Photometry of late-type stars with inhomogeneous surface brightness distributions revealed distinct light-curve variations modulated with the stellar rotation period. Numerous papers dealt with this phenomenology since the early seventies, starting with the first interpretations by Kron (1947, 1952), Hoffmeister (1965), Chugainov (1966), Catalano & Rodonó (1967) and Hall (1972) and finally culminating in the use of fully automatic telescopes for continuously monitoring RS CVn- and even solar-type stars (e.g. Henry et al. 1995a). Recently, Hall (1994) presented an update of the total number of known spotted stars and listed 357 entries including the chromospherically-active binaries from the catalog of Strassmeier et al. (1993a).
The physical phenomena behind such photometric modulations are starspots, i.e. local magnetic fields that are strong enough to prevent the overturning convective motion and thus the flow of energy from the stellar interior outwards to the surface and consequently appear as cool and therefore dark areas against an otherwise bright photosphere (Biermann 1941). The simple existence of starspots enables not only an accurate determination of the stellar rotation period, and eventually also differential rotation, but also to obtain some information about the surface temperature distribution and its intrinsic changes. We believe that these changes on the surface are a signature of the underlying dynamo process, itself rooting in the deepest convective layers of the star. Armed with long time series of photometric data one could try to detect analoga of the solar 11-year spot cycle in evolved stars or in pre-main sequence objects or even discover a stellar analog of the solar butterfly diagram. An automated approach is the key towards the needed time and phase coverage.
| Star | V | Spectral type | Binary? |  |  |
 | Radius | Variable type |
| (mag) | (SB/S) | (days) | (days) | (kms-1) | ( ) | |||
| HD 12545 (XX Tri) | 8.1 | K0III | SB1 | 23.98 | 24.30 | 17 |  8.2 | RS CVn |
| HD 17433 (VY Ari) | 6.9 | K3-4IV-V | SB1 | 13.20 | 16.42 | 6 | 1.95 | RS CVn |
| HD 26337 (EI Eri) | 7.0 | G5IV | SB1 | 1.947 | 1.945 | 50 | 1.9 | RS CVn |
| HD 283518 (V410 Tau) | 11.0 | K4 | S | ... | 1.872 | 77 | 3.0 | weak T Tauri |
| HD 283750 (V833 Tau) | 8.2 | K5V | SB1 | 1.788 | 1.797 | 6.3 | 0.22 | BY Dra |
| HD 282624 (SU Aur) | 8.9 | G2 | S | ... | 2.98 | 66 | 3.9 | class. T Tauri |
| HD 31738 (V1198 Ori) | 7.1 | G5IV | SB2 | ? | (4.59) | 19 | 1.7 | RS CVn |
| HD 31993 (V1192 Ori) | 7.5 | K2III | S | ... | 6.78 | 31 | 4.1 | single giant |
| HD 39576 | 9.1 | G1V | S | ... | 2.7 | 20 | 1.07 | solar type |
| HD 81410 (IL Hya) | 7.5 | K1III | SB1 | 12.90 | 12.73 | 24 | 6.0 | RS CVn |
| HD 82443 | 7.0 | K0V | S | ... | 5.43 | 5 | 0.54 | solar type |
| HD 82558 (LQ Hya) | 7.8 | K2V | S | ... | 1.606 | 28 | 0.94 | solar type |
| HD 106225 (HU Vir) | 8.6 | K0III-IV | SB1 | 10.39 | 10.1 | 25 | 5.1 | RS CVn |
| HD 111812 (31 Com) | 4.9 | G0III | S | ... | ? | 57 | ? | single giant |
| HD 112313 (IN Com) | 8.0 | G5III-IV | SB1 | ? | 5.9 | 67 | 7.8 | RS CVn |
| HD 113816 | 8.3 | K0III | SB1 | 23.7 | 23.5 | 5-10 | 2.3-4.6 | RS CVn |
| HD 116544 (IN Vir) | 9.2 | K2-3IV | SB1 | 8.19 | 8.23 | 23.5 | 3.8 | RS CVn |
| HD 117555 (FK Com) | 8.1 | G2III | S | ... | 2.40 | 159 | 7.3 | FK Comae |
| HD 129333 (EK Dra) | 7.5 | G0-1V | S | ... | 2.70 | 17.5 | 0.92 | solar type |
BD-08 3999 (UZ Lib) | 9.3 | K0III | SB1 | 4.77 | 4.74 | 67 | 6.3 | RS CVn |
| HD 195040 (AT Cap) | 8.9 | K2III | SB1 | 23.21 | 23.2 | 19 | 10 | RS CVn |
| HD 202077 (BM Mic) | 8.3 | G6IV-V | S | ... | 14.3 | ? | ? | single subgiant |
| HD 216489 (IM Peg) | 5.6 | K2II-III | SB1 | 24.65 | 24.4 | 24 | 12 | RS CVn |
|
|
In the past decade only few photometric studies were presented that tried to
continuously monitor spotted stars and then model their intrinsic spot
variations from one rotation to the next (e.g. Bartolini et al.
1983; Strassmeier et al. 1988; Jetsu et al.
1990;
Olàh et al. 1991; Strassmeier & Bopp 1992;
Strassmeier et al. 1994a; Henry et al. 1995b;
Rodonò et al. 1995; Olàh et al. 1996). One physical
parameter that can be extracted from these data is the spot lifetime. For
example, Henry et al. (1995b) observed
individual spot lifetimes on 
And and II Peg between
0.5 years and over 6 years, on 
Gem between 0.2 and 3.5 years and on
V711 Tau between 0.4 and 4 years from 15-19 years of photometric
observations. Similar was found for two other active stars, VY Ari = HD 17433
(Strassmeier & Bopp 1992) and HR 7275
(Strassmeier et al. 1994a) or for HK Lac where polar activity centers persisted for over
30 years (Oláh et al. 1997).
Similar timescales are encountered in RS CVn itself:
from 45 years of photometric data Rodonó et al. (1995) found a
cyclic change of the total spotted area with a period of 20 years and a
spot minimum in 1993 and a maximum in 1984-88. Although spot lifetimes are
not explicitly given in that paper we estimate a lower limit of about
0.4 years from their spot maps in 1991 and again in 1993.
On the other hand, the
G5IV-binary EI Eri exhibits spots with very short variability timescales,
sometimes as short as several stellar rotations or about two weeks
(Strassmeier 1990).
Variable | Comparison | Check | Time range |
 |  |  |
| 2400000+ | ||||||
| HD 12545 (XX Tri) | HD 12478 | SAO 55178 | 48648 - 50130 | 302 | 259 | 58 |
| HD 17433 (VY Ari) | HD 17572 | HD 17361 | 48529 - 50137 | ... | 315 | 72 |
| HD 26337 (EI Eri) | HD 26409 | HD 25852 | 49284 - 50123 | 5 | 270 | 23 |
| HD 283518 (V410 Tau) | HD 27159 | HD 27570 | 49612 - 50164 | 134 | ... | ... |
| HD 283750 (V833 Tau) | HD 283749 | HD 29169 | 48957 - 49353 | 231 | ... | 25 |
| HD 282624 (SU Aur) | HD 31565 | HD 31305 | 49403 - 49443 | 14 | ... | |
| HD 31738 (V1198 Ori) | HD 31594 | HD 32263 | 49044 - 50174 | 316 | ... | ... |
| HD 31993 (V1192 Ori) | HD 32073 | HD 32191 | 49023 - 50172 | 286 | ... | ... |
| HD 39576 | HD 39636 | SAO 170938 | 49023 - 50182 | 271 | ... | ... |
| HD 81410 (IL Hya) | HD 81904 | HD 80991 | 49023 - 50230 | 440 | ... | ... |
| HD 82443 | HD 83098 | HD 83821 | 49393 - 50170 | ... | 142 | ... |
| HD 82558 (LQ Hya) | HD 82447 | HD 82508 | 49044 - 50233 | 445 | 40 | ... |
| HD 106225 (HU Vir) | HD 106270 | HD 1057961 | 48245 - 50263 | 581 | 809 | ... |
| HD 111812 (31 Com) | HD 111469 | HD 111395 | 49405 - 50263 | 104 | 197 | ... |
| HD 112313 (IN Com) | HD 112299 | HD 112706 | 49044 - 50263 | 521 | ... | 80 |
| HD 113816 | HD 113449 | SAO 139146 | 49025 - 50263 | 564 | ... | ... |
| HD 116544 (IN Vir) | HD 117635 | HD 118830 | 49441 - 50263 | 451 | ... | 30 |
| HD 117555 (FK Com) | HD 117567 | HD 117876 | 49044 - 49892 | 249 | ... | 58 |
| HD 129333 (EK Dra) | HD 129390 | HD 129798 | 49398 - 50263 | 210 | 112 | ... |
|
BD-08 3999 (UZ Lib) | SAO 140587 | SAO 140589 | 49022 - 50263 | 688 | ... | ... |
| HD 195040 (AT Cap) | HD 195106 | SAO 189362 | 49082 - 50261 | 393 | ... | ... |
| HD 202077 (BM Mic) | HD 202540 | HD 201695 | 49101 - 50263 | 241 | ... | ... |
| HD 216489 (IM Peg) | HD 216635 | HD 2166722 | 49297 - 50263 | 218 | ... | 111 |
|
|
1after JD 2450000 we used HD 106332 as the check star.
2this is the variable star HR Peg = HR 8714 (see discussion).
Obviously, photometric monitoring with a high duty cycle and for a long period in time is needed and the major boost for continuous starspot photometry came only with the availability of fully automatic photoelectric telescopes. Fairborn Observatory was the first to put such telescopes into operation in 1983 (Boyd et al. 1984) and a first series of papers was published by the then Vanderbilt/Fairborn group of D.S. Hall (Strassmeier & Hall 1988a,b; Strassmeier et al. 1989a; Boyd et al. 1990). In the meantime altogether nine APTs, eight in the U.S. and one in Italy, are currently taking data on various types of objects (see several articles in Henry & Eaton 191995). Repeated observations are also made at several manual observatories, most noticeably at ESO with the 50-cm size telescopes (Sterken 1983; Sterken et al. 1995; Cutispoto 1995). The results of all these photometric observations are series of light curves for about hundred spotted stars with coverages just interrupted by the annual seasons, the day-and-night cycle and, of course, unfavorable weather conditions and - rare - telescope failures.
In this paper we present new multicolor data from the Fairborn
0.75-m T7 APT (now part of the University of Vienna twin
APT; see Strassmeier et al. 1997), the 0.25-m Phoenix-10 APT
(Boyd et al. 1984) and from the Catania 0.8-m APT
(Rodonó &
Cutispoto 1994). Altogether 23 program stars were observed
in either UBV or 
or in both and also with varying degrees
of time and phase coverage on the individual telescopes.
Table 1 (click here) presents our target stars and
their commonly known stellar parameters. A period
analysis for all stars for each individual observing season as well as
for the combined data gives a homogeneous set of stellar rotation
periods and their seasonal variations. The long-term light curve behavior
is examined together with additional, published photometry. Partially
unpublished data by Schnell (1996), Oláh & Jurcsik
(1996), Jurcsik (1996), Skiff &
Nations (1991),
Hall (1988), Barksdale (1990), and Petreshock (1996) are also used in the analysis in this paper.