COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3228

                                                         Konkoly Observatory
                                                         Budapest
                                                         15 August 1988

                                                         HU ISSN 0374 - 0676


          PHOTOMETRIC VARIABILITY OF HD81410 IN 1988 *

The single-line chromospherically active spectroscopic binary HD 81410
has a relatively large orbital period P = 12.86833 days and its colors at the
maximum light level correspond to a system with K0 III primary and F5 V 
secondary (Raveendran et al., 1982). Bidelman and MacConnel (1973) detected strong
emissions in Ca II H and K. Eggen's (1973) photometric observations of
HD 81410 during 1971 and 1972 showed light variability with an amplitude of
0.50 mag and he estimated the period to be 25.4 days. The 1981 photometry
by Raveendran, et al. (1982) and their subsequent analysis of the photometry
and spectroscopy, available till then, showed that the system has an orbital
period of 12,86833 days and 12.89 days periodicity in light variation. The
shape and the amplitude of light curve in 1981 was entirely different from
that of 1971 and 1972. These variations are caused by the rotational
modulation of magnetically active regions, spots, that are cooler than the
surrounding photosphere. Slee et al. (1984) detected radio emission from
HD 81410 at 5 GHz and also they observed flux variability by a factor of
2 in 90 minutes during a major flare of 1983 August 2.
   HD 81410 was observed with the 50 cm telescope at the European Southern
Observatory, La Silla, on 18 nights; 7 nights through UBVRI filters and a
cooled RCA 31034 photomultiplier tube and 11 nights through Stroemgren uvby
filters and a cooled EMI 6256 tube. The measurements were made differentially
with respect to the comparison star HD 81904. Sufficient numbers of UBVRI
and uvby standard stars were observed for the conversion of the instrumental
magnitudes to the standard system. The mean error in V is of the order of
0.006 mag and in y 0.007 mag. The Stroemgren y is nearly the same as the
Johnson V, and in order to compare both sets of observations, the y magnitudes
were converted to Johnson V values using the relation

       V = y + 0.015 (b-y) -0.003 (Olsen, 1983).


                            [FIGURE 1]


   The orbital phase were calculated according to the ephemeris of Reveendran
et. al. (1982):

       JD = 2441466.213 + 12.86833d E

   Figure 1 is a plot of V magnitudes. The rectangles represent V magnitudes
of UBVRI photometry and open circles the calculated V values from Stroemgren y.
The light curve is asymmetric with a flat minimum (0.60P to 0.85P) and has an
amplitude of 0.16 mag. The magnitudes at the maximum and minimum are 7.25 mag
and 7.41 mag respectively,
   From the photometry of HD 81410 available in the literature, it is seen that
the light curve during 1971/72 was quasi-sinusoidal with an amplitude of 0.45
mag, minimum at 0.70P and the maximum light level 7.50 mag, In 1981 the light
curve had two minima and the maximum brightness 7.50 mag while the amplitude
decreased to 0.15 mg. The unpublished observations obtained by Mekkaden during
1987 indicated that the light curve had two minima; the deeper minimum
occurring at the same phase as that of the light curve in 1988  This means
that the large spot group responsible for the deeper minimum in 1987 was
present in 1988 while the small spot group of 1987 disappeared. The unchanged
maximum light levels of these two seasons reveal that the activity on the less
spotted hemisphere remained unchanged.
   The changes in the light curve and the mean light level occur due to the
formation and disintegration of active regions. Mekkaden and Geyer (1988)
interpreted such types of activity due to the evolution of short lived spot
groups in active zones on stellar photosphere and the cumulative effect of
these spot groups cause changes in the activity in these stars.

* Based on observations collected at the European Southern Observatory La
Silla, Chile;


    M.V. MEKKADEN 1,2      D. SINACHOPOULOS 1


1,  Auf dem Huegel 71
    5300 Bonn, F.R. Germany

2, Indian Institute of Astrophysics
   Bangalore, India



References:

Bidelman, W.P, and MacConnel, D.J., 1973, Astr. J., 78, 687. [BIBCODE 1973AJ.....78..687B ]
Eggen, O.J., 1973, Publ, astr. Soc. Pacif., 85, 42. [BIBCODE 1973PASP...85...42E ]
Olsen, E.H., 1983, Astron. Astrophys. Suppl., 54, 55. [BIBCODE 1983A&AS...54...55O ]
Mekkaden, M.V. and Geyer, E.H., 1988, Astron. Astrophys., 195, 214. [BIBCODE 1988A&A...195..214M ]
Raveendran, A.V., Mekkaden, M.V., and Mohin, S., 1982, Mon. Not. R. astr. Soc.
     199, 707. [BIBCODE 1982MNRAS.199..707R ]
Slee, O.B., Haynes, R.F., and Wright, A.E., 1984, Mon. Not. R. astr. Soc.
     208, 865. [BIBCODE 1984MNRAS.208..865S ]