COMMISSION 27 OF THE I. A. U.
       INFORMATION BULLETIN ON VARIABLE STARS
                   Number 2109

                               Konkoly Observatory
                               Budapest
                               1982 March 17

                               HU ISSN 0374-0676





            29 DRACONIS: A NEW VARIABLE STAR



  We suspected 29 Draconis = HD 160538 is an RS CVn binary because of
its late spectral type, the Ca II H & K emission in its spectrum, and its
apparently variable radial velocity. The spectral type is K0 III according
to Bidelman (1955) or K2 III according to Roman (1955), and "strong bright
lines of Ca II are present" according to Bidelman (1955). Radial velocity
values published by Abt and Biggs (1972) show variations larger than we
felt could reasonably be blamed on observational uncertainty; except for
these variations, however, we can find no direct evidence that 29 Dra is a
binary system.
  Photometry was begun to search for the variability which might be 
expected if 29 Dra were an RS CVn binary. We observed it at four different
observatories on a total of 47 different nights between JD 2 444 295.97 and
2 444 556.49 in 1980. Henry observed with the 24-inch Seyfert reflector at
Dyer Observatory and the No. 4 16-inch reflector at Kitt Peak National
Observatory; Louth observed with his 11-inch reflector in Sedro Woolley,
Washington; Renner observed with his 10-inch reflector at Scuppernong
Observatory near Dousman, Wisconsin. All observed with filters selected to
match V of the UBV system, although Henry obtained also a few observations
at Dyer with a filter selected to match B. All used BD +75d647 as the 
comparison star. The individual differential magnitudes, corrected for 
differential atmospheric extinction with mean coefficients appropriate for each
observatory and transformed differentially to the UBV system with coefficients 
determined previously, have been deposited in the I.A.U. Commission
27 Archive for Unpublished Observations of Variable Stars (Breger 1979),
where they are available as file no. 94. In the reductions we used a mean
color difference of Delta(B-V) = +0.08m, in the sense variable minus comparison.
  The light curve in V is shown in the figure below, where asterisks are
Henry at Dyer, circles are Henry at Kitt Peak, and plusses are Louth. Each

point is a mean of the three individual differential magnitudes obtained on
each night, although the earliest point Delta V = 0.332m at  JD 2 444 295.97 By
Renner) is not plotted. Here we can see that 29 Dra is very definitely
variable with an overall range of Delta V = 0.12m a periodicity of around 30
days, and a long-term brightening trend.

                          [FIGURE 1]

To determine this periodicity more precisely, we used a simple program
which searched for the period which Produced a magnitude vs phase plot in
which the total length of line. segments connecting the points was a minimum.
The result was P = 31.5d with an uncertainty we estimate to be around +-1
day. Times of maximum and minimum light, estimated graphically from the
figure, are consistent with this period. A representative epoch of minimum
light would be JD 2 444 445.0 =- 1.0d.
   In virtually all RS CVn binaries known to be variable in light, the
photometric period is within a few percent of the orbital period, as a 
consequence of synchronous rotation of the star responsible for the variation.
Therefore we conclude this note by suggesting spectroscopic observers search
for radial velocity variations in 29 Dra and predict that the period of
those variations, if found, will be around 30 days. For such a bright star,
V = 6.55m according to Roman (1955), this should not be difficult.

    D.S.H, is  happy to acknowledge support from N.A.S.A. research grant
NSG- 7543,




              DOUGLAS S. HALL ^a
              GREGORY W. HENRY ^a 
              Dyer Observatory
              Vanderbilt University
              Nashville, Tennessee 37235

              HOWARD LOUTH ^b
              2199 Hathaway Road
              Sedro Woolley, Washington 98284

              THOMAS R. RENNER
              Scuppernong Observatory
              4512 Deerpark Drive
              Dousman, Wisconsin 53118

              STEVEN N. SHORE
              Warner and Swasey Observatory
              Case Western Reserve University
              Cleveland, Ohio 44106





References:
Abt, H. A. and Biggs.; E. S. 1972. Bibliography of Stellar Radial Velocities
   (Tucson: Kitt Peak National Observatory). [BIBCODE 1972bsrv.book.....A ]
Bidelman, W. P. 1955, Ap, J. Suppl. 1, 175. [BIBCODE 1954ApJS....1..175B ]
Breger, M. 1979, I.B.V.S. no. 1659.
Roman, N. G. 1955, Ap. J. Suppl. 2, 195. [BIBCODE 1955ApJS....2..195R ]





a) Guest Observer, Kitt Peak National Observatory, operated by the 
   Association of Universities for Research in Astronomy, under contract 
   with the National Science Foundation.

b) of the A.A.V.S.O.


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