COMMISSION 27 OF THE I. A. U.
                  INFORMATION BULLETIN ON VARIABLE STARS
                              Number 3563

                                                        Konkoly Observatory
                                                        Budapest
                                                        14 February 1991

                                                        HU ISSN 0374 - 0676


                  THE LONG TERM BEHAVIOUR OF MWC 560

   The emission line star MWC 560 was discovered by Merrill and Burwell (1943).
It shows an interesting spectrum with P-Cygni profiles, widened Balmer
absorption lines and an M-Type spectrum in the red region. In the ultraviolet
the star shows a strong continuum. In the U-band flickering with an, amplitude
of ~0.2 mag had been observed (Bond et al. 1984).
   The observed properties led to the assumption that MWC 560 is a symbiotic-
like object.
   Although this star might be an interesting object, it has been forgotten by
the astronomers in recent years. Early in 1990, MWC 560 attracted attention by
remarkable rise in brightness. The star has been observed photoelectrically and
spectroscopically by Tomov et al. since January 1990. The general rise in
brightness is superimposed by rapid variations with an amplitude of some tenth
of a magnitude. The spectrum also varies. Remarkable is the presence of strong
blue shifted (up to -6000 km/h) Balmer lines with strongly varying profiles
(Tomov et al. 1990a). Lack of observational data makes these phenomena
difficult to explain.
   I studied the photometric behaviour of MWC 560 on about 750 plates of the
Sonneberg sky patrol covering the epoch from 1928 to the beginning of 1990
(see figure 1).
   The star shows a Z And-like lightcurve with long time-scale variations of
about 3 mag. There were two outbursts prior to J.D. ~2437000. Then the shape
of the lightcurve changed. The brightness rose by about 2 mag during about
1000 days. The decrease to the quiet light came off much more slowly within
about 4000 days. The process repeated itself, but with a short flesh during
decrease at J.D. ~2444000. Smaller variations of short period have
occasionally been observed. This kind of variability was absent during the
great outbursts in the past. Since J.D. ~2444750 the star rapidly gained in
brightness and is now about 3 mag brighter than during its quiet stage.


                              [FIGURE 1]

              Figure 1  Photographic lightcurves of MWC 560


   Tomov et al. discussed a model of this star (Tomov et al. 1990b).  They
assume a symbiotic object consisting of an M-giant and a hot compact companion
(e.g. a white dwarf). There is an accretion disk around it because of mass
transfer from the giant to the hot star. This assumption is supported by the
short-period variations in brightness with small amplitude. On account of the
early constant radial velocities of the emission lines they assume that we see
the disk "face on". In that case the accretion disk contributes most of the
light, especially in the shorter wavelength range. Non-stable mass transfer
from the M-giant might produce the long time-scale variations in brightness
because of a variable accretion disk.

   Without detailed spectroscopic and photometric studies it will be impossible
to understand this interesting object.


                                            R. LUTHARDT

                                            Sonneberg Observatory
                                            O-6400 Sonneberg/Thür.
                                            Germany



References:

Bond, H.E., et al., 1984, B.A.A.S. 16, 516 [BIBCODE 1984BAAS...16..516B ]
Merrill, P.W., Burwell, C.G., 1943, Astrophys. J. 98, 153 [BIBCODE 1943ApJ....98..153M ]
Tomov, T., et al., 1990a, Nature  346, 637 [BIBCODE 1990Natur.346..637T ]
Tomov, T., et al., 1990b, "The puzzling star MWC 560", poster paper at the
      12th European Regional Meeting, Oct. 1990, Davos, Switzerland
Tomov, T., 1990, private communication