COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                     Number 1934

                                   Konkoly Observatory
                                   Budapest
                                   1981 March 11

                                   HU ISSN 0374-0676



 PHOTOMETRIC OBSERVATIONS OF THE SHORT PERIOD W UMa SYSTEM
                        BM UMa


BM UMa has been discovered as a variable star by Hoffmeister
(1963) and classified as "short period". Meinunger and Wenzel
(1968) classified it as RRc star with a period of 0.136d.
Shugarov (1975), however, found that BM UMa is an eclipsing
binary of W UMa type.
     The star has been observed photoelectrically in B and
V with the double beam photometer at the 1.06m telescope of
Hoher List Observatory on February 22, 1980. Comparison star
has been an 11th magnitude (V) star with the approximate 
coordinates (1950) 11h09.2m, +46d43'. Minima times have been
obtained:
           Min. I  JD hel. 2444292.3496
           Min. II JD hel. 2444292.4853
The period given by Shugarov was not precise enough to link
the new minima with his minimum epoch. By adding 0.068d also
the maxima determined by Meinunger and Wenzel have been used
to derive the new elements:
        Min. I = JD hel. 2437348.558 + 0.2712207d E
They satisfy all cited extrema.
     Figs. 1 and 2 show the light curves in both colours.

                       [FIGURE 1]

          Fig. 1   B light curve of BM UMa

                       [FIGURE 2]

          Fig. 2   V light curve of BM UMa

  The formation of normal points and a preliminary rectification 
lead to the conclusion that the eclipses are complete.
Although the maxima differ somewhat in brightness, major
asymmetries of the light curve seem to be lacking. By the
large photometric errors of this 14th (V) magnitude object
it could only be estimated (on the basis of the Russell-
Merrill method and the Anderson and Shu (1979) theoretical
light curve atlas) that the ratio of the radii is of the
order of 0.8 to 0.9 and the orbital inclination is about
85d. The broad maxima resemble light curves with a fill-out
f approx. 0.5.
    It should be checked if the well observed photographic
light curve by Meinunger and Wenzel, which led to the 
classification "RR Lyrae star", is an indication for a variable
asymmetry in the light curve of BM UMa.





                         M. HOFFMANN
                   Observatorium Hoher List
                    5568 Daun / Eifel, BRD


References:
Anderson, L., and Shu, F. H., 1979, Astrophys. J. Suppl. Ser.
          40, 667 [BIBCODE 1979ApJS...40..667A ]
Hoffmeister, C., 1963, Astron. Nachr. 287, 169 [BIBCODE 1963AN....287..169H ]
Meinunger, L., and Wenzel, W., 1968, Veroff. Sternw. Sonneberg
          7, Heft 4, 389 [BIBCODE 1968VeSon...7..385M ]
Shugarov, S. Yu., 1975, Astron. Zirk. 888, 7 [BIBCODE 1975ATsir.888....7S ]