COMMISSION 27 OF THE I. A. U.
        INFORMATION BULLETIN ON VARIABLE STARS
                     NUMBER 885

                                Konkoly Observatory
                                Budapest
                                1974 April 30

            AY Mus: A TRIPLE SYSTEM?


The eclipsing binary AY Mus was observed photographically in
1924-1931 (Refs. 1,2) and was found to have equal minima about
0.3 magnitudes deep. The secondary minimum fell at phase 0.48,
thus the orbit is eccentric with e>=0.03. In January 1973 however,
photoelectric UBV-observations with ESO's 50 cm telescope at 
La Silla failed to show any eclipses deeper than 0.02 magnitudes.
A closer examination of the records possibly shows the
primary minimum near the expected phase, giving the improved
ephemeris

            JD_pr= 2441683.80 + 3.20555d E
                       +/-.01 +/-.00001

The secondary minimum could not be found in these observations:
it may be (if observable) anywhere in the phase-interval 0.45-0.55.
In Fig. 1. are shown the present B light-curve and the old photographic
one. The present observations were made relative to the comparison 
stars in Table 1; no variations greater than 0.01 magnitude were 
found for them.

            Table 1. Measured UBV-values

                    CPD      HD (E)   Sp (HD)    V      B-V     U-B
AY Mus            -64d1669   310592      B9    10.31    0.18   -0.14
Comp 1            -64d1677   100638      A0     7.16    0.10   -0.35
Comp 2            -64d1682   101174      B8     7.41    0.09   -0.44

The only tenable explanation of the present absence of eclipses is
a third body in the system, causing nodal regression and consequently
changed inclination to the plane of the sky for the eclipsing
pair. Reasonable assumptions about the dynamics of the system
leads to an expected period for the third body in the interval
100d < P_3 < 900d.

                       [FIGURE 1]

   Figure 1. The light curve of AY Mus from photographic
   estimates compared to the new photoelectric observations.
   Ordinate unit 0.1 magnitudes. Sizes of symbols
   proportional to their weight.

A more detailed discussion of the observations and their interpretation
is to be published elsewhere. Meanwhile, further observations
of AY Mus are of course desirable: photometric to study changes in
the depths of minima; spectroscopic in order to possibly reveal the
third star, either by its spectrum or by its influence on the 
barycentric velocity of the close pair (the velocity amplitude may be
about 10 km/s). Also, any old photographic plates taken near minimum
light would be valuable in tracing the development of the light-curve.


                           STAFFAN SODERHJELM
                           Lund Observatory
                           S-22224 Lund, Sweden.


References:
1) Utterdijk J., 1932, BAN 6, 295. [BIBCODE 1932BAN.....6..295U ]
2) Oosterhoff P.Th., and van Houten, C.J., 1949, BAN 11, 63. [BIBCODE 1949BAN....11...63O ]