The most efficient period determination method is through the organization of multi-site campaigns in which more than three or four observatories are involved. This has provided long and continuous data series which, contrary to past techniques, provide reliable frequency sets that correctly describe the nature of the stars. However, long before this current fashion was begun, several multi-site observational campaigns were devoted to securing data of short period, small amplitude Delta Scuti variable stars. These data, although not as extensive as those produced by modern campaigns, serve to verify the frequency sets that have been recently found in the longer campaigns. In this sense, the data of the present paper permits discrimination of several sets of frequencies that have been found for each of the stars considered.
The advantages of studying variable stars in a given cluster are mainly that the member stars are at the same distance and that the parameters that fix the evolution of the stars, such as age and chemical composition, can be considered to be the same for all the stars within the cluster. These parameters, although generally poorly determined or known indirectly for only a few stars, are well known for nearby clusters and, jointly with the mass and the effective temperature, allow a better determination of the description of the pulsation mechanisms. This is the main reason that makes the study of variables in open clusters important since the differences shown will, in principle, throw light on the causes that provoke the triggering mechanism of the pulsations. On the other hand, observations of variable stars in a cluster are difficult to carry out since these stars exhibit short periods and very small amplitudes.
Because of the relative nearness of Praesepe (NGC 2632, M 44, C0837+201) to the Sun, this cluster has been, and still is, the subject of many studies of all the different kinds of stars that constitute it. See, for example, Pinfield et al. (1997) and the references within it for a summary of its properties. Proper motion work has been extensive, and has led to an excellent list of "member" stars. Klein Wassink (1927), Vanderlinden (1923), Schrick (1953), & Artiukhina (1966) presented the preliminary most extensive studies on the topic, and the star numbers assigned by the first author have been used in most subsequent studies.
The distance modulus m-M = 6.0 given by
Johnson (1957) was derived by
overlapping the main sequence in the color-magnitude diagram for Praesepe on the
main sequence of the same diagram in the Hyades.
Later, Crawford & Barnes (1969) with four-color and H
photometry measured 97 stars in the
Praesepe cluster; they determined an average distance modulus of 6.1 for the
cluster, based also on the comparison to the Hyades' Main Sequence. Later,
Nicolet (1981), using data from the Jenkins (1952)
General Catalog of
Trigonometric Parallaxes (GCTP) determined distance modulii for 43 open
clusters; a value of 
was obtained for Praesepe. According to Hambly et al. (1995)
the most
recent determination of the Praesepe distance modulus appears to be that of
Reglero & Fabregat (1991) with a value of 6.05 mag, although they mention that
a large scatter in the distance modulus determination by several authors is
found.
Johnson & Knuckles (1955) have noted that the space motion of the
Praesepe cluster is essentially the same as for the Hyades. They assigned an age
of 
yr for both clusters and they also noted other similarities
such as metal abundances and ultraviolet excess. Harris (1976)
gives an age of
yr. Later, Anthony-Twarog (1982)
estimated an age of 
yr; Lang (1991) reports an age of 
yr. From the
compilation of Tsvetkov (1989) for ages of several open clusters, a mean value
for Praesepe of 
yr is found.
Dickens et al. (1968) made an extensive study taking into account all the available
data of four-color photometry to clarify the rotational velocity effects on the
absolute magnitude. A correlation between Johnson's (1952)
(U-B) and V sin i was
confirmed and their estimation on rotational velocity is one of the most
complete for this cluster. For the original detection of the Delta Scuti
variable stars, see the paper by Breger (1972) and the subsequent papers.
With the development of new observational techniques, knowledge of open clusters has increased due to the incredible quantity and quality of the information gathered. The recent works of Mermilliod et al. (1990), who initiated their study in 1978 with the Coravel determination of radial velocities of F5-K0 dwarf stars and UBVRI (Kron) photometry have allowed them the identification of 43 new members of the halo. A second, very extensive study, to determine membership is that of Jones & Stauffer (1991) which was based on proper motions and photometry of the cluster. They presented 765 probable members from V = 9 to 18 and calculated a luminosity function. The same aim was pursued by Hambly et al. (1995) extended to low mass main sequence membership. ICCD speckle observations of binary stars in Praesepe has been done recently by Mason et al. (1993). They listed one new binary star, KW 212, out of over 54 potentially short period stars and three more: KW 203, KW 265 and KW 284, that were discovered before. In particular, KW 284 is very important for the present work because at times it has been used as a comparison star.
Several studies on the detection of Delta Scuti stars in some nearby open
clusters have been carried out. These studies were done principally
by Breger (1969, 1970, 1972a,b) in the Pleiades, the Hyades, Praesepe, and Coma;
later this type of search was carried out by Slovak (1978) in 
Persei
and by Horan (1979) in the Hyades. These authors have found that about 30% of
the Main Sequence cluster stars inside the instability strip show detectable
variability. Table 1 (click here) gives some of the characteristics of all the Delta Scuti
stars found up to now in the Praesepe Cluster. Rodriguez et al.
(1994) have
summarized the photometric characteristics of the observed stars.
The values of the amplitude, 
, and the period p, have been taken
from this source.
| KW | HD | GCVS | V | | P (d) | b-y | m1 | c1 | | SpT |
| 045 | 73175 | BR Cnc | 8.26 | 0.02 | 0.038 | 0.136 | 0.201 | 0.865 | 2.790 | A9V |
| 114 | 73345 | CY Cnc | 8.14 | 0.04 | 0.0325 | 0.121 | 0.209 | 0.888 | 2.813 | F0V |
| 154 | 73450 | BS Cnc | 8.50 | 0.02 | 0.051 | 0.149 | 0.197 | 0.793 | 2.770 | A7V |
| 204 | 73575 | BT Cnc | 6.66 | 0.04 | 0.1023 | 0.153 | 0.180 | 0.995 | 2.778 | F0III |
| 207 | 73576 | BU Cnc | 7.68 | 0.02 | 0.056 | 0.104 | 0.199 | 0.969 | 2.812 | A6V |
| 284 | 73712 | - | 6.78 | 0.004 | 0.149 | 0.161 | 0.180 | 0.937 | 2.756 | A9V |
| 292 | 73729 | BQ Cnc | 8.19 | 0.198 | 0.0973 | 0.01 | 0.172 | 0.809 | 2.742 | F2V |
| 318 | 73746 | BV Cnc | 8.65 | 0.02 | :0.21 | 0.181 | 0.197 | 0.749 | 2.748 | A9V |
| 323 | 73763 | BN Cnc | 7.80 | 0.03 | 0.0353 | 0.130 | 0.189 | 0.900 | 2.796 | A7V |
| 340 | 73798 | BW Cnc | 8.48 | 0.01 | 0.072 | 0.147 | 0.213 | 0.809 | 2.764 | F0V |
| 348 | 73819 | EP Cnc | 6.78 | 0.01 | 0.1829 | 0.091 | 0.195 | 1.075 | 2.818 | A6Vn |
| 385 | 73890 | - | 7.91 | 0.01 | 0.0383 | 0.144 | 0.196 | 0.855 | 2.791 | A7V |
| 445 | 74028 | BX Cnc | 7.97 | 0.02 | 0.053 | 0.120 | 0.201 | 0.911 | 2.812 | A7III |
| 449 | 74050 | BY Cnc | 7.92 | 0.01 | 0.058 | 0.115 | 0.197 | 0.947 | 2.812 | A7V |
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