COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                      Number 900

                                   Konkoly Observatory
                                   Budapest
                                   1974 June 22

    FIELD-SCANNING PHOTOELECTRIC OBSERVATIONS OF THE
           ECLIPSING VARIABLE YY Gem


   Photoelectric observations of the 10th magnitude eclipsing
variable YY Gem (Castor C) taking into account scattered 
background light from the nearby star Castor A (alpha Gem) have been 
carried out in UBV using the 36-inch reflectors of the Dodaira Station
of Tokyo Astronomical Observatory and the Okayama Astrophysical
Observatory on five nights during the winter of 1973-74.
BD+31d1627 was used as the primary comparison star, with occasional
light constancy checks provided by observing BD+31d1611.
   Difficulties in measurement are due to the strong scattered
light of alpha Gem wich is 73" distant from the variable studied.
Scattered light may amount to roughly 8%, 25% and 50% of the system
light in V,B and U respectively. However, by careful scanning
of the background sky in the vicinity of the variable it has been
possible to determine valid light levels at various observation
times.

                           [FIGURE 1]

  The accompanying diagram shows a typical scan in U light from
which a representative light level of the variable may be found. The
scan is carried out by moving the telescope in declination starting from
some point distant from the Castor system, moving until the star under 
investigation (YY Gem) is in the centre of the diaphragm, and proceeding
onward in the direction of alpha Gem so that the character
of the sky background increase can be examined. The telescope
may be continuously moved in the same direction past the vicinity
of the bright stars until the sky background again subsides to a
normal value. In this way the symmetrical distribution of the scattered
light may be checked and provide a verification of the sky
background level during the scan of the variable.
   The scanning direction of declination was chosen so that the
scattered background light would show a monotonic increase as 
indicated in the diagram. In the right-ascension mode this pattern
of variation is not so clear, due to the northerly position angle
of the bright star (alpha Gem) with respect to the variable studied
(YY Gem).
   This approach has not been carried out before. In Kron's study
of the system (Ap.J., 115, 301, 1952) the problem of the scattered light
was not clarified. We therefore expect to obtain definite improvements
over earlier observations of the system as our results so far suggest.
   It has been established that primary and secondary eclipses of
almost equal depths (= 0.50m in V) occur, with eclipses lasting for
about 2 hours. The eclipse may be slightly deeper in B and U light.
From the present observations, the mean out-of-eclipse magnitude in
V light is V=9.27m, while B-V = 1.46m and U-B = 1.4m. It is hoped to
continue observations of the system in the coming winter.
   On the nights of Jan. 9-10, 11-12, 12-13 of 1974 continuous
photoelectric monitoring was also carried out with the synchronous
UBV photometer attached to the same telescope at Okayama to find
any flare-like phenomenon as reported by Moffett and Bopp (Ap.J.,
168: Letter 117, 1971). However, no flare-like phenomenon could be
detected during these three nights.




                   E. BUDDING and M. KITAMURA,
                   Tokyo Astronomical Observatory,
                   Japan.