COMMISSION 27 OF THE I.A.U.
              INFORMATION BULLETIN ON VARIABLE STARS
                           Number 1424

                                                     Konkoly Observatory
                                                     Budapest
                                                     1978 May 12


          OPTICAL PULSES IN HD 153919 = 3U1700-37

An O6f supergiant star HD 153919 is the counterpart of the
eclipsing X-ray source 3U1700-37 with the binary period 3d.4 (Jones
et al. 1973). The optical light curve shows two shallow minima. The
secondary optical minimum coincides with the X-ray eclipse (Penny
et al. 1973, Jones and Liller 1973, van Paradijs et al. 1978). 
Recently the X-ray pulses with a period of 97 +- 1.5 min (0d.0674 +-
0d.0010) were discovered by Matilsky and Jessen (1978). The optical
variations of HD 153919 on the time scale of one to three hours
had been already reported by van Genderen and Uiterwaal (1976) and
by Kemp (1973).
   This note presents results of a search for 97 min optical
pulsations in an extensive set of the five-colour photometric 
observations of HD 153919 published by van Paradijs et al. (1978).
   The observations made with the yellow filter have been 
examined for presence of the 97 min variability. An upper limit of
about 0.002 mag can be set for the amplitude of such variations in
observations obtained during the X-ray eclipse. Outside the X-ray
eclipse the periodic pulsations are of very small amplitude, if
present at all, with an exception of the orbital phases 0.44 - 0.55
when the periodic variations are clearly visible and attain an
amplitude of 0.02 mag. These phases correspond to the primary 
optical minimum which occurs when the X-ray source is seen in front
of the hot companion star. There are observations from four nights
which fall in this range of phases. For these four nights it was
possible to phase the short term variability with a period of
0d.06678 which is comfortably close to the X-ray period. A normal
time of the light maximum is J.D. 2443024.272.

                       [FIGURE 1]

   Fig. 1 presents the average light curve of the short term
variations in HD 153919 obtained from observations with yellow filter 
for the interval of the orbital phases 0.44 - 0.59. If observations 
from all orbital phases are used then this light curve
is again well defined but with 5-fold decrease in the amplitude.
If other filters are used we obtain very similar light curves. A
slight dependence of the amplitude and/or the time of the maximum
on the wavelength is possible. Because the short term variability
persists only during limited time every 3d.4 days many aliases can
fit the observations almost as well as the period derived. The
closest among aliases is a period of 0d.06809 which is also consistent 
with the X-ray period. It is very likely that the pulsation
period of this object decreases quickly with time (Matilsky and
Jessen 1978, Ziolkowski 1978). The optical observations have been
obtained 200 days earlier than the X-ray data, therefore the 0d.06809
or even longer period is plausible for the considered set of the
photometry.
   There is another set of the photometric data (van Genderen
and Uiterwaal 1976) taken 400 days earlier than the observations
of van Paradijs et al. The short term variability during the primary 
optical minimum is clearly seen in Fig. 1 in the van Genderen
and Uiterwaal paper. An attempt to determine possible periods
from their data resulted in values 0d.06638, 0d.06770 and 0d.06907.
Neither of them coincides with any period that can be fitted to
the van Paradijs et al. data. A tentative picture is the period
of 0d.06907 for the van Genderen and Uiterwaal data, then 400 days
later 0d.06809 and finally still 200 days later 0d.0674 +- 0d.0010
from the X-ray data. A faster decrease of the period is also possible.
   The presence of the optical pulses and especially their
confinement to the narrow range of the orbital phases should be
explained. This can be tentatively done by assuming that the primary 
optical minimum is at least partly due to a transit in front
of the optical component of the semi-transparent cloud of gas
situated around the X-ray source. This cloud of gas may have form
of an accretion disc, or an accretion wake. The X-ray pulsations
modify periodically the physical conditions and the velocity field
in the nearby gas and this in turn influences the transparency of
the gas cloud and consequently the fractional light loss and the
optical brightness of the system.


                                A. KRUSZEWSKI

                                Warsaw University Observatory
                                al. Ujazdowskie 4
                                PL OO-478 Warszawa



References:

van Genderen, A.M., and Uiterwaal, G.M. 1976, Astr. and Ap., 52, 139 [BIBCODE 1976A&A....52..139V ]
Jones, C., Forman, W., Tananbaum, H., Schreier, E., Gursky, H.,
     Kellog, E., and Giacconi, R. 1973, Ap. J. Letters, 181, L43 [BIBCODE 1973ApJ...181L..43J ]
Jones, C., and Liller, W. 1973, ibid. 184, L65 [BIBCODE 1973ApJ...184L..65J ]
Kemp, J.C. 1973, ibid. 185, L21 [BIBCODE 1973ApJ...185L..21K ]
Matilsky, T., and Jessen, J. 1978, IAU Circ., 3193 [BIBCODE 1978IAUC.3193....4M ]
van Paradijs, J.A., Hammerschlag-Hensberge, G., and Zuiderwijk,
     E.J. 1978, Astr. and Ap. Suppl., 31, 189 [BIBCODE 1978A&AS...31..189V ]
Penny, A.J., Olowin, R.P., Penfold, J.E., and Warren, P.R. 1973,
     Monthly Notices R.A.S., 163, 7P [BIBCODE 1973MNRAS.163P...7P ]
Ziolkowski, J. 1978, private communication