COMMISSIONS 27 AND 42 OF THE IAU
             INFORMATION BULLETIN ON VARIABLE STARS
                          Number 4008

                                            Konkoly Observatory
                                            Budapest
                                            24 March 1994

                                            HU ISSN 0374 - 0676


           A PHOTOMETRIC STUDY OF THE NEW ECLIPSING BINARY HD 21155

 HD 21155 is an eighth magnitude B8 star that was discovered to be a 3.045d eclipsing
binary by Kaiser (= DHK 9) some five years ago (Baldwin & Kaiser 1989). Very little
is known about the system but visual observations reveal a primary minimum of approx. 0.4
mag and the secondary minimum is not seen. Photoelectric observations around phase
0.5 (Kaiser et al. 1990) put an upper limit on the secondary minimum of approx. 0.03 mag,
so it is possible that the true period is approx. 6 d. However, the visual and photographic
observations show no differences between alternate minima.
 New photoelectric observations of this system have been made on 47 nights during
1991 to 1993 from Catsfield, East Sussex in southern England with a 25-cm Newtonian
equipped with a prototype JEAP (EMI9924B PMT based) photon-counting photometer
(Walker 1986, 1991) and computer controlled data acquisition. All the 
observations were
made with a nominal V filter and integration times were 30 seconds through a 1 arc minute
aperture. The comparison and check stars were HD 21193 and HD 21099 respectively and
the sequence, comparison - sky - variable - check was repeated between 2 (one night only)
and 6 times for each observation. The mean Delta m magnitudes, variable minus comparison
and comparison minus check, and their errors were calculated. The standard deviation of
the comparison minus check star measurements (both stars are fainter than the variable)
is 0.026 mag.
  The magnitude differences, variable - comparison, are plotted in Figure 1, using the
ephemeris
             HJD_I = 2435988.336 + 3.0452976 x E

given by Kaiser et al. (1990). The plot shows a well defined primary minimum of approx.
0.44 mag and strong indication of secondary minimum of approx. 0.04 mag. If the secondary
eclipse has been detected then it confirms that the period given in the ephemeris is
correct, as opposed to twice that value. Unfortunately all the observations in the core
of primary minimum are from the same (6 day) minima, so it is not possible to provide
the additional confirmation of identical alternate minima. Nevertheless, the secondary
minimum, although small, seems well established. The time of primary minimum derived
from these observations is

                 2449049.621 +/- 0.007,

and has an O-C of 0.004 days with respect to the ephemeris. Although this timing is
arguably the most accurate one available, it does not make any significant improvement
to the ephemeris, which should be more than adequate for some years.
  An attempt has been made to model the system using Hill's LIGHT2 code. The details
will be published elsewhere (Lloyd & Watson 1994) but the results suggest that both
components are evolved. The secondary is cool, T_e probably < 5000 K, underluminous
and probably fills its Roche lobe. The solution plotted in Figure 1 is for a slightly-evolved
B8 primary with R = 5.0 R_sun and a Roche lobe filling secondary of 1.5 M_sun, with R = 4.6
R_sun and T_e = 5000 K. The standard deviation of the residuals from this line is 0.026 mag,
which is consistent with the observational errors. The evolved, cool, low-mass secondary
clearly indicates that this is an Algol binary.


                            [FIGURE 1]

 Figure 1. The light curve of HD 21155 from the new photoelectric data. The open
 symbols indicate the less reliable data, where the s.e. of the mean Delta m's
 > 0.015 mag.


  At present there are no radial velocities of this system, but if it is at all similar to the
solution presented here then the spectroscopic orbit, should have an easily detectable K1
approx. 70 km/sec. Additional photoelectric observations of rather higher precision than those
presented here will be necessary to improve the details of the light curve, but the most
vital requirement is for a radial velocity solution.


                       J. WATSON
                       Iddons, Henley's Down, Catsfield,
                       Battle, East Sussex TN33 9BN, UK

                       C. LLOYD
                       Astrophysics Division,
                       Rutherford Appleton Laboratory, Chilton,
                       Didcot, Oxon. OX11 0QX, UK


References:

Baldwin, M. E., Kaiser, D. H, 1989, JAAVSO, 18, 1 [BIBCODE 1989JAVSO..18....1B ]
Kaiser, D. H., Baldwin, M. E., Williams, D. B., 1990, IBVS, No. 3442
Lloyd, C., Watson, J., 1994, Obs., submitted [BIBCODE 1995Obs...115...75L ]
Walker, E. N., 1986, JBAA, 97, 30 [BIBCODE 1986JBAA...97...30W ]
Walker, E. N., 1991, Variable Star Research: An International Perspective (Eds. J.R.
      Percy, J.A. Mattei & C. Sterken, Cambridge University Press) p. 122