COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                    Number 2396

                                   Konkoly Observatory
                                   Budapest
                                   5 September 1983

                                   HU ISSN 0374-0676


 SPECTROSCOPIC OBSERVATIONS OF THE COOL ECLIPSING X-RAY BINARY

                    HD155638

   The determination by Bloomer et al. (1983) of the orbital
period and eclipse epoch of the cool active X-ray source,
HD155638 (Stern et al. 1981), permits evaluation of the phases
of my spectrograms obtained at the Lick Observatory on 14
nights. The spectrum is dominated by the lines of the cooler
component, for which my estimate of the spectral type is in
agreement with the G8IV estimated by Stern et al. The
distribution of phases is poor for orbit determination, and on
only 6 of the nights are the weaker lines of the hotter 
component seen in the photographic or D-line region of the spectrum.
Even at maximum separation, the lines of the components are
not well resolved, and velocities of the hotter component are
uncertain. The strong emission lines of CaII give velocities
in agreement with those of the cooler component. The lines
of the hotter star are too weak and poorly resolved for an
evaluation of its spectral type, although from the relative
appearance of the D lines and FeI lines, it must be an F star.
No evidence is seen of hydrogen lines in this star.
   The values of K, the velocity amplitudes, are 46 km s^-1
for the cooler star and a roughly estimated 50 for the hotter.
These values, together with the period, 27.55d, lead to masses
1.3 and 1.2 m_Sun, typical for double-lined RS CVn systems. The
radii, for i=90d, obtained from the widths of eclipse, are
8.2 and 2.5 R_Sun. The latter is somewhat larger and the former
much larger than for the components in other RS CVn eclipsing
binaries (e.g., Popper 1980), and this system appears more
evolved than the others. For the shorter periods, the more
massive, cooler components cannot become as large as 8 R_Sun
because of Roche lobe constraints. HD155638 may be considered
an RS CVn system intermediate between typical double-lined
systems such as RS CVn and WW Dra, with their cooler components
subgiants having radii about 4 R_Sun, and single-lined systems
such as lambda And, in which the cooler component has evolved into
a larger giant (15-20 R_Sun?) so luminous that lines of the
hotter component are not seen in the spectrum.

   The photometry of Bloomer et al. (1983) leads to some
contradictions. From the depths of total eclipse in V, 0.23
mag, the magnitude difference between the components at the
time of conjunction is DeltaM_V=1.6 mag, the cooler component being
more luminous. The magnitude difference at this phase can
also be computed from the ratio of the radii and the ratio of
luminous surface fluxes. The former, obtained from the
phases of inner and outer eclipse contacts, is <=3.3, the upper
limit being for i=90d. The ratio of surface fluxes in V may
be evaluated as follows. From the reported depths of total
eclipse in V, B, and U (0.23, 0.43, and 0.76 mags), the color
differences are Delta(B-V)=0.79, Delta(U-B)=0.80. It is difficult
to match these color differences with the estimated spectral
types. If the types were, for example, G8IV and F5, as
estimated, the differences would be only 0.38 and 0.47 mags.
The observed color differences would be matched approximately
by F0 and K1III or IV, although both, these types appear
somewhat outside the range admitted by the appearance of the
spectrograms. Combining the fluxes corresponding to the latter
types (e.g., Popper 1980, Table 1) with the ratio of the radii
leads to Delta M_V ~0.0, far from the value 1.6 obtained from the
eclipse depth in V. To match the latter, the ratio of radii
would have to be much greater than the upper limit or the
difference in spectral type would have to be considerably
less than required by the depths in V, B, and U. In order to
achieve Delta M_V=1.6 with R_c/R_h =3.3, a flux ratio corresponding
to types G0V and G8IV, for example, would be required. But
types this close together are completely outside the range
permitted by the color differences derived from the eclipse
depths in the three wavelength bands.
    Thus there are two apparent contradictions: The differences 
between the components in B-V and U-B are greater than
would be expected from the appearance of the spectra, and,
perhaps more serious, the difference in absolute magnitude
between the components is in serious disagreement when
evaluated in two different ways, namely from the depth of
total eclipse on the one hand and from the ratio of the radii
and ratio of surface fluxes, the latter derived from the
difference in color index, on the other. Improved photometry
leading to better values of the widths and depths of eclipse,
and perhaps scanner spectrophotometry as well, would be required
to examine these matters further.





               D. M. POPPER

               Department of Astronomy
               University of California
               Los Angeles





References:

Bloomer, R. H., Hanson, W. A., Fried, R. E., Hall, D. S., and
  Henry, G. W., Ap. J. Lett., 270, L79. [BIBCODE 1983ApJ...270L..79B ]
Popper, D. M., 1980. Ann. Rev. Astr. Astrophys., 18, 115. [BIBCODE 1980ARA&A..18..115P ]
Stern, R. A:, Nousek, J. A., Nugent, J. J., Agrawal, P. C.,
  Riegler, G. R., Rosenthal, A., Pravdo, S. H., and
  Garmire, G. P., Ap. J. Lett., 251, L105. [BIBCODE 1981ApJ...251L.105S ]