Be stars have long been known to be photometric variables. These
variations can have periodic and/or irregular components of different time
scales: (a) periodic or multiperiodic components with small amplitudes of
roughly several hundredths of magnitude in the V magnitude, characterized by
short time scales 
to 3 days (Balona 1990, 1995)
which are commonly
interpreted as due to non-radial pulsations and/or to stellar surface spots or
corotating features (Baade & Balona 1994); (b) periodic variations with
intermediate time scales, 
days to 100 days, with amplitudes 
of several hundredths or tenths of magnitude
(Harmanec & Kříž 1976) which are observed only in binary Be stars; (c) quasi-cyclic or irregular
variations with rather long time scales, months, years and sometimes decades,
generally implying magnitude changes that range between 0.01
and 0.4
mag (Feinstein 1970, 1975; Feinstein & Marraco 1979;
Alvarez & Schuster 1981;
Kozok 1985; Percy et al. 1988). The latter can, however, be as high as 
to 1.2 mag (Huffer 1939; Mook et al. 1974;
Howarth 1979; Alvarez &
Schuster 1981; Bernacca & Bianchi 1981;
Divan et al. 1982, 1983; Mennickent &
Vogt 1991; Apparao 1991; Roche et al. 1993;
Mennickent et al. 1994; Ballereau et al. 1995).
Though the irregular fluctuations are, as a rule, characterized by
long time scales, these scales have been found in the case of very active Be
stars to be as short as a few days (Hubert-Delplace et al. 1982). The
quasi-cyclic or irregular photometric variations of Be stars are accompanied by
more general spectrophotometric variations which concern a wide wavelength
coverage. These variations have also been known for a long time
(Chalonge & Safir 1936; Greaves & Martin 1938;
Barbier & Chalonge 1941). Since Gerasimovic (1929);
Chalonge & Safir (1936); Barbier & Chalonge (1939),
it is also known
that when line emission or "shell'' features are pronounced, Be stars can
present two Balmer discontinuities (hereafter BD). The first BD looks like that
of a normal B star and it has the same spectral location. The second BD appears
at shorter wavelengths, which means that it is formed in a medium with a lower
pressure than in the photosphere of the central star. It can be in emission
(during a strong emission line phase) or in absorption (during a strong
"shell'' or absorption line
phase). The value of the second BD in emission not only depends on the spectral
type (Peton 1981) but is correlated with Balmer emission line strength
(Schild 1978; Feinstein & Marraco 1979;
Peton 1981; Kaiser 1989;
Dachs et al. 1989) and
with Balmer decrement as well (Divan et al. 1982). The emission component of the
BD is generally not higher than -0.15 dex, but may exceptionally be as high as
-0.25 dex (Barbier 1947). When the second BD is in absorption, it can
sometimes represent a flux deficiency stronger than 1 mag (Zorec 1986, 1994).
The second BD disappears during a faint emission/shell line phase, so that the
stellar continuum energy distribution near the BD looks like that of a normal B
star. During the spectrophotometric variations of Be stars, the first BD
maintains the value and mean spectral position measured during an emissionless
phase to closer than 0.02 dex and 
Å respectively
(Divan & Zorec 1982a,b). The constancy of the first BD to within 0.03 dex was
also confirmed by Dachs et al. (1989) and it is currently used for spectral type
classification (Divan 1979; Divan & Zorec 1982c;
Divan et al. 1982, 1983).
Therefore, it is also useful to determine the effective temperature of the
central object (Divan & Zorec 1982c; Zorec et al. 1983;
Zorec 1985, 1986;
Kaiser 1989; Dachs et al. 1989;
Zorec & Briot 1991) and the surface gravity
(Divan & Zorec 1982a; Zorec 1985, 1986).
It has been shown that the irregular photometric variations imply a global
continuum flux excess or deficiency as compared to stellar underlying
photospheric radiation. This can be estimated as 
, where 
(V magnitude of the
star+envelope system; V* magnitude of the star alone) and it is 
for continuum emission (or flux excess) and 
for
continuum flux deficiency. The magnitude excess is positively
correlated with Balmer line emission strength
(Zorec & Briot 1985, 1991; Dachs
et al. 1988, 1989; Ballereau et al. 1995)
and with reddening of the Paschen
continuum (Divan et al. 1978; Divan 1979;
Chkhikvadze 1980; Dachs 1982). In
turn, this reddening is correlated with Balmer line emission strength
(Dachs et al. 1988). The visible flux deficiency is generally small (
mag) and it is not accompanied by marked changes of visible Paschen energy
distribution (Divan & Zorec 1982c; Zorec et al. 1989;
Zorec 1994). Quasi-cyclic
and/or irregular changes from positive to negative flux excesses are symptomatic
of the known Be "phase'' variations Be 
B-normal
Be-shell. The present paper is devoted to the irregular
spectrophotometric variations of Be stars.
The existing observational studies on long-term photometric and/or spectrophotometric variations of Be stars can be classified into two broad groups: (1) studies where authors used observations of their own; (2) studies based on compilations of published observations each made in a given photometric or spectrophotometric system.
(1) - The most relevant studies in the first group are those of
Pavlowski et al. (1996) for northern Be stars and
Manfroid et al. 1991, 1995;
Sterken et al. 1992, 1993 for southern ones. Observations presented by
Pavlowski et al. (1996), done in the UBV system, concern the monitoring of 76 Be stars over 18
years since 1972. In this program there are 28 well-observed stars with
9+8-3 observing seasons and each with 392+805-288 individual
observations, and 48 less observed objects with 3+2-2 observing seasons
each and with 60+159-57 individual UBV measurements.
Manfroid's et al. 1991, 1995 and
Sterken's et al. (1992, 1993) 12-year observing program also
concerns, among other variable stars, 15 Be stars which were monitored in the
uvby Strömgren system since 1982 for about 6 months per year and with a time
frequency between 2 and 14 days. From Pavlovski's et al. (1996)
study it appears
that the number of stars with variations on time scales from months to years is
about the same as the number with time scales of days or fractions of days.
These variations are, however, 3 times more frequent among stars earlier than B5
than in the later ones. Six stars present orbital components of photometric
variations and only 2 show sudden light changes. From the published light
curves, which in many cases reveal complicated variations difficult to
summarize, we can distinguish those with shell-type characteristics, where two
dominant behaviours seem to appear: (i) those of stars where the (U-B) colour
index changes much less than the (B-V) index; (ii) those, the most numerous,
where the (U-B) colour index changes much more than the (B-V) index. Both
types of behaviours are seen in early as well as in late type Be stars.
Mennickent et al. (1994) report a detailed study on 13 Be stars based on uvby
Manfroid & Sterken's et al. 1991-1995 observations. These authors note: (a)
quasi-periodic oscillations on time scales between 10 and 20 days with
amplitudes up to 0.2 mag preceding fading events; (b) in 7 stars, which mostly
have shell characteristics, they observe that: (b1) small or rather no
variations of the b-y index accompany the variation of magnitude u; (b2)
there is a correlation between index c1 and magnitude u; (c) in all 13
studied Be stars and for all uvby magnitudes, there is, within the
uncertainties of coefficients defining the regression lines, a mean relation
between the long-term variation amplitudes and the standard deviation of
short-term variability. Concerning the (c1,u) relation, it is worth noting
that in shell Be stars not only the variation of u is noticeably higher than
in v but also 
, so that the correlation between c1 and u
seems natural. However, the well-defined trend between the slope of the
(c1,u) relation and 
, which identifies possible differences in the
relative behaviours of Balmer to Paschen continua, could still deserve further
attention.
The first group also comprises studies of long-term spectrophotometric variations of Be stars made using the BCD system (for definition of the BCD (Barbier-Chalonge-Divan) system, see Chalonge & Divan 1952). The great advantage of this system is that it reflects the variation of continuum energy distribution only, because it is not affected by spurious "reddening/blueing'' effects introduced by variable spectral lines entering the wide wavelength intervals of some photometric filter pass-bands. The BCD data were obtained at irregular time intervals over more than 40 years. They concern a small number of Be stars, of which most are of early spectral types. These data led Divan (1979); Divan et al. (1982); Zorec (1986) to conclude that in the studied objects the continuum emission phases were characterized by well-defined relations between the visible gradients of energy distributions, total Balmer discontinuity and visual magnitude. These relations seem to be different for each star and unchanged even after the star had a phase of apparent loss of emission characteristics. These authors also found that, in general, reddening of the visible gradient is positively correlated with stellar brightening. Inversely, "shell'' phases are characterized by very small changes, if any, in the visible energy distribution.
(2) - In the second group are papers concerning either a small number of
objects at typical "Be'' phases, or a large number of objects at any variation
phase. In the first subgroup, works like Dachs (1982);
Dachs & Hanuschik (1984)
and Dachs et al. (1988) show that long-term photometric variations can be
characterized by slopes 
ranging between about -0.1 and
-0.3 for individual stars and that reddening of the (B-V) index is related
to stellar brightening. In the second subgroup, it is worth mentioning
Nordh & Olofsson (1977); Hirata (1982) and
Hirata & Hubert-Delplace (1981). From
UBV data of 50 to 70 Be stars spread over more than 10 years, Hirata et al.
concluded: (i) that early type Be stars get redder when they brighten, and on
the contrary late type Be stars are bluer when they brighten; (ii) the slope
is steeper when is higher, and that this
slope has the same sign as the colour excess ratio 
.
Finally, there are some attempts of a theoretical nature to explain the
long-term photometric variations of Be stars. Using a cylindrical model for the
CE, Hirata (1984) concluded that changes of V and (B-V) are due to
variations of the extent of the circumstellar envelope (hereafter CE) and that
is a function of spectral type and aspect angle. The
irregular variations of V magnitude were also discussed by
Zorec & Briot (1991). They concluded that when 
, the irregular variations may
correspond to two different opacity regimes of the circumstellar envelope: (a)
a low opacity regime, where variations of V are mostly due to small changes of
opacity, and (b) a high opacity regime, where variations of V are mainly due
to changes of the extent of the CE. Inversely, for 
there is only
a high opacity regime. In a recent work, Hirata (1995) studied the long-term
variations of some Be stars at shell phases and concluded that their variations
are produced by extended photospheres.
As continuum and line variations correlate only when strong line emissions or line absorptions are present during "spectroscopic'' Be and Be-shell phases respectively, to avoid misunderstandings relative to spectroscopic or spectrophotometric variations, whenever necessary we explicitly refer in this paper to "spectrophotometric Be phase'' (hereafter SPh-Be) when the second BD is in emission and to "spectrophotometric shell phase'' (hereafter SPh-shell) when the second BD is in absorption.
Results presented in the preceding section are consistent and complementary
to each other. The relations obtained between photometric colour indices and/or
spectrophotometric parameters show that they should somehow depend on the
physical structure of the CE and on its changes. In particular, early BCD
results suggested that these relations are permanent for each star and differ
from one object to another, as would be the case if there were permanent
characteristics related to the structure of the CE of Be stars which subsist
even after phase changes or apparent losses of emission. However, in addition
to opacity and geometrical factors cited in the preceding section, still other
phenomena could be responsible for photometric/spectrophotometric changes of Be
stars. In fact, as the contributing regions in the CE to the
variations are expected to be located rather near the underlying star
(Zorec & Briot 1991; Zorec & Garcia 1991;
Cruzado et al. 1994) ionization/excitation
balance changes in the deepest layers of the CE (Magnan 1979), as well as
radiation transfer effects related to non-thermal phenomena
(Gebbie & Thomas 1970, 1971; Thomas 1983)
should also be relevant. Due to all these effects and
because some of the above observational results were obtained from a small
number, mostly early type Be stars, it would still be necessary: (a) to analyse
a larger sample of Be stars of all possible subspectral types, each having been
observed as frequently as possible and over a long time base; (b) to study the
actual character (constancy, uniqueness, etc.) of relations among
spectrophotometric parameters for each star, especially, if feasible, for those
having undergone phase changes. This study aims at giving as far as possible
comprehensive observational bases on long-term spectrophotometric variations of
Be stars, derived from compiled photometric and spectrophotometric data obtained
in various systems, to ensure a large time base coverage and for a quite a
large number of stars.