COMMISSION 27 OF THE I. A. U.
              INFORMATION BULLETIN ON VARIABLE STARS
                          Number 2538

                                              Konkoly Observatory
                                              Budapest
                                              18 June 1984

                                              HU ISSN 0374-0676


                        PHOTOMETRY OF CAPELLA

  At the suggestion of Dr. D. Hall we decided to measure Capella (=alpha Aur =
BS 1708) for possible variability. We present here a small sample of photometric
measurements, made differentially in the standard manner (Hall and
Genet, 1982) with respect to 9 Aur (= BS 1637). For alpha Aur, B-V = +0.80, for
9 Aur, V = 5.00, B-V = +0.34 (see Table 9 of Johnson et al., 1966). The 1980
data were reduced with a standardization coefficient epsilon = -0.072. These
data were reduced from ammeter readings. Observations were also made on four
other nights in 1980, but the differential magnitudes showed a range greater
than 0.050 mag, indicating non-uniform sky conditions or problems with the
equipment. The 1981 data were made with a different photomultiplier tube (for
which epsilon =-0.047), and were reduced from trip chart tracings, In all cases
an atmospheric extinction coefficient of k = 0.5 mag/air mass was used. The
differential extinction corrections in all cases amounted to less than 0.02 mag,
The observations were made with a 6-inch f/6 reflector in San Jose, California,
a standard V filter, and an uncooled RCA 931A tube operated at -1000 V.
  Data in Table I includes date, Universal Time, geocentric Julian Date,
  
                           Table I

                 Photometry of alpha Aur vs. 9 Aur

Date               UT       Julian Date      Delta V       n
                            (2440000+)
7/8 Feb 1980     0511         4277.7161   -4.998+-.008     4
9/10 Mar 1980    0446         4308.6986   -4.999+-.010     3
12/13 Mar 1980   0420         4311.6803   -5.034+-.006     3
21/22 Mar 1980   0359         4320.6662   -5.020+-.005     3
4/5 Jan 1981     0423         4609.6824   -5.004+-.006     3
21/22 Feb 1981   0410         4657.6738   -5.039+-.008     3
28/29 Mar 1981   0422         4692.6822   -5.009+-.011     2

nightly means, and number of measurements.
 The average of all 21 differential measurements is Delta V = -5.014 +-0.004
(standard deviation of the mean). There is no statistically significant
difference between the 1980 and the 1981 data.
 This small data set indicates that Capella exhibits no variability greater
than +-0.02 mag in V. Given the standardized V magnitude of 9 Aur, Capella has
V = -0.01. This is brighter than the value given by Johnson et al.,
(V = +0.08). Some of this discrepancy can be attributed to uncertainties in
the calibration of gain settings.


                  K. KRISCIUNAS
                  UKIRT
                  900 Leilani St.
                  Hilo, HI 96720
                  U.S.A.

References:

Hall, D.S. and Genet, R.M., 1932, Photoelectric Photometry of Variable Stars,
  chapters 11-13
Johnson, H.L. et al., 1966, Communic. Lunar and Planetary Lab. 4, 99 [BIBCODE 1966CoLPL...4...99J ]