COMMISSION 27 OF THE I. A. U.
            INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3690

                                                  Konkoly Observatory
                                                  Budapest
                                                  10 December 1991

                                                  HU ISSN 0374 - 0676

   Light variability of HD 224639, HD 224638 and HD 224945 *


The short period light variability of HD 224639 (7.3, F0) was first detected by
Manfroid and Renson (1983) who used it as a comparison star in a programme for
the search for Ap variables. Since it was probable that the star could be a
delta Scuti type variable, we included it in our programme devoted to the study
of multimode pulsators.

We observed it in the B colour with the 50-cm telescope at Merate Observatory
for 3 nights in 1988 and 6 nights in November 1989. During this second run we
also observed the star during 9 nights with the ESO 50-cm telescope at La Silla
Observatory. These observations showed that HD 224639 is a typical delta Scuti
star with a maximum amplitude of about 0.13 mag in the B colour, and a
preliminary frequency analysis of the data indicated the presence of several
pulsation modes. The data were in any case unable to allow an unambiguous
frequency identification (Mantegazza and Poretti, 1990).

Using HD 224638 (7.2, F0) and HD 224945 (6.9, A3) as comparison stars, and 
observing them with the same frequency as HD 224639, we discovered that at least
one of them was variable with an amplitude of some hundreths of a magnitude and
a characteristic time scale of the order of several hours. For these reasons
we planned new intensive observations that we performed at La Silla with the
ESO 50-cm telescope from September 5 to October 9, 1991. We were able to get
data during 20 nights, observing consecutively for several hours (up to 8) on
many of them. In order to detect which of the two comparisons was variable we
adopted a third comparison star, HD 225086 (8.0, F2), but after the first
observing night it was evident that both HD 224638 and HD 224945 were variable,
so that in order to check the constancy of HD 225086 and the accuracy of the
measurements a fourth comparison star was introduced in the observing cycle.
During the successive 4 nights we used HR 11 (6.43, B8 IIIp), but since the
colour of this star is consistently different from that of the others, with the
need of introducing large colour terms in the reduction procedure, we preferred
to drop it and use in the following nights HD 200 instead. In any case HD
225086, HR 11 and HD 200 were found to be constant within a few thousandths of
a magnitude. In the Figure, we show the light curves obtained during a typical
night. From top to bottom we have plotted the magnitude differences of HD
225086 with respect to HR 11, HD 224945, HD 224638 and HD 224639 respectively
(in the sense "HD 225086 minus program star"). We can see that while HD 224639
shows the typical lightcurve of a delta Scuti star, with an apparent


                              [FIGURE 1]


periodicity of the order of 0.1 days, HD 224638 and HD 224945 have a slow
variation with timescales longer than the observational baseline. A
preliminary examination of the complete set of light curves of these two stars
shows the non-regular nature of this variation even if characteristic time
scales could probably be defined. F type stars showing the same kind of
variability have been discovered with increasing frequency in the last years
(Abt et a1.,1983; Antonello & Mantegazza, 1986; Krisciunas & Guinan, 1990;
Mantegazza et al., 1991) however careful studies are still missing and the
nature of the light variability is still elusive. We hope that our conspicuous
datasets will enable us to derive the statistical properties of the light
variations of HD 224638 and HD 224945. Regarding HD 224639 we have about 2700
differential magnitudes in the B colour with respect to HD 225086,
corresponding to about 130 observational hours. A preliminary frequency
analysis of these data shows a complex pulsation spectrum with 4 modes, at
6.21, 9.49, 9.54 and 10.57 c/d, having about the same semi-amplitudes of
roughly 15 mmag, and with many more of lower amplitudes.


                     L. MANTEGAZZA ^1, E. PORETTI ^2

                     1-Dipartimento di Fisica Nucleare e Teorica
                       Universitá di Pavia
                     2-Osservatorio Astronomico di Brera
                       Via Bianchi, 46
                       22055 Merate - Italy

References:

Abt, H.A., Bollinger, G., Burke, E.W.: 1983, Astrophys.J. 272, 196 [BIBCODE 1983ApJ...272..196A ]
Antonello, E., Mantegazza, L.: 1986, Astron.Astrophys. 164, 40 [BIBCODE 1986A&A...164...40A ]
Krisciunas, K., Guinan, E.: 1990, Inf.Bull.Var.Stars No. 3511
Manfroid, J., Renson, P.: 1983 Inf.Bull.Var.Stars No. 2311
Mantegazza, L., Poretti, E.: 1990, in Confrontation between stellar pulsation
  and evolution", ASP Conference Series Vol. 11, p.324 [BIBCODE 1990ASPC...11..324M ]
Mantegazza, L., Poretti, E., Antonello, E.: 1991, Inf.Bull.Var.Stars No. 3612

* Based on observations made at the European Southern Observatory (ESO), La
Silla, Chile