COMMISSION 27 OF THE I. A. U.
             INFORMATION BULLETIN ON VARIABLE STARS
                          Number 2571

                                              Konkoly Observatory
                                              Budapest
                                              3 August 1984

                                              HU ISSN 0374-0676


             RADIAL VELOCITY VARIATIONS IN THE
               Ca II EMISSION STAR HD 36705

  We present radial velocity measurement which suggest that the star
HD 36705, previously thought to be single, is in fact a single lined binary.
  HD 36705 (K1 IIIp, Houk and Cowley, 1975) shows strong Ca II H & K
emission, and was suggested as a possible RS CVn star (Weiler and Stencel,
1979). However Collier (1982) found no radial velocity variations in ten
spectrograms of this star, and concluded that it is probably a member of the
FK Comae class of rapidly rotating single late type giant (Bopp and Stencel,
1981). Pakull (1981) identified HD 36705 with a flaring X-ray source observed
by the EINSTEIN satellite. He also obtained extensive photometric
observations, finding a period of 0.51423 +- 0.00005 day, and a light curve
which changes significantly on short time scales. Rucinski (1983) obtained
further photometry, confirming the short period and the rapidly varying light
curve. Collier et al.(1982) and Slee et al.(1984) report a possible detection
of this star at radio frequencies.
  We obtained 14 spectra of HD 36705 at a dispersion of approximately
10 A/mm with the Cassegrain echelle spectrograph on the 1.0 m telescope of
the Australian National University at Siding Spring Observatory on 1984
February 16, 19, 20 and 21. The detector was the Mount Stromlo red two
dimensional Photon Counting Array. All data reduction was done with the Mount
Stromlo PANDORA reduction program.
  We also made eleven measurements of four stars whose radial velocities
are accurately known from the work of Griffin (1972). The mean deviation of
our results from those of Griffin is -0.9 km/s, with an rms error of 2.5 km/s.
The measured radial velocities of HD 36705 are likely to be less
precise, due to the high rotational broadening (v sin i = 70 +- 10 km/s,
Collier, 1982).
   Only about 0.8 of the approximately half day period was covered by our
observations, with only about 0.6 well covered. The data indicate a radial
velocity variation of at least 20 km/s.
   We combined our data with those previously published by Collier (1982).
We retained the epoch of Pakull (1981) of HJD 2444296.575, but the combined
radial velocity data spanning four years are better fitted by a period of
0.51425 day. Our derived period is consistent with that of Pakull within his
quoted uncertainty.
   The radial velocity data are plotted in Figure 1, (open circles:
Collier, 1982; closed circles: our results) with the epoch and period given
above. A best fitted sine wave is also plotted; the equation is

            V = 36.9 + 10.9 sin 2pi(phi - 0.137)

where phi = phase as computed from the given epoch and period.


                               [FIGURE 1]


    Although the data are scattered, we believe the variations to be real. We
suggest that Collier's (1982) conclusion can be explained by the fact that this
measurements were made when the velocity was not changing greatly with phase.
   The amplitude of the line of sight orbital velocity is about 10 km/s
and the line of eight rotational velocity is 70 +- 10 km/s (Collier, 1982).
Assuming synchronous rotation, these results imply that the centre of mass of
the system is well within the visible component and that the mass ratio q
is < 0.1.
   The large rotational velocity of the primary implies that it cannot be a
giant, because of equational break up; thus the secondary must be a very low
mass object.
   Hearnshaw (1979) obtained eleven spectra of this star, and suggested that
radial velocity changes may be present. However, no numerical results were
given, so we cannot compare Hearnshaw's results with our data.
   Figure 2 is a photoelectric V light curve obtained during 1984 February


                               [FIGURE 2]


to April, with the Monash Observatory 0.25 m telescope, using a 1P21 photo-
multiplier tube. All measurements were made differentially using HD 37297 as
a comparison star, as used by Pakull (1981). These data are plotted with the
same epoch and period as Figure 1. The results suggest that minimum light
occurs near radial velocity maximum. If the photometric variations are due to
dark starspots then maximum spot visibility must also occur at this orbital
phase.
  We are preparing a fuller account of this work for publication
elsewhere. We thank Mount Stromlo and Siding Spring Observatories for access
to the 1.0 m telescope and other facilities.


           J.L. INNIS, D.W. COATES and K. THOMPSON
           Department of Physics, Monash University
           Clayton, Victoria, Australia, 3168

References:

Bopp, B.W. and Stencel, R.E., 1981. Astrophys. J., 247, L131. [BIBCODE 1981ApJ...247L.131B ]
Collier, A.C., 1982. Mon. Not. R. astr. Soc., 200, 489. [BIBCODE 1982MNRAS.200..589C ]
Collier, A.C., Haynes, R.F., Slee, O.B., Wright, A.E. and Hillier, D.J., 1982.
   Mon. Not. R. astr. Soc., 200, 869. [BIBCODE 1982MNRAS.200..869C ]
Griffin, R.F., 1972. Mon. Not. R. astr. Soc., 155, 449. [BIBCODE 1972MNRAS.155..449G ]
Hearnshaw, J.B., 1979. IAU Colloq. No. 46, 371. [BIBCODE 1979ctvs.conf..371H ]
Houk, N. and Cowley, A.P., 1975. Michigan Spectral Catalogue, Vol. 1.
   University of Michigan. [BIBCODE 1975mcts.book.....H ]
Pakull, M.W., 1981. Astron. Astrophys., 104, 33. [BIBCODE 1981A&A...104...33P ]
Rucinski, S.M., 1983. Astron. Astrophys. Suppl. Ser., 52, 281. [BIBCODE 1983A&AS...52..281R ]
Slee, O.B., Haynes, R.F. and Wright, A.E., 1984. Accepted for publication in
   Mon. Not. R. astr. Soc. [BIBCODE 1984MNRAS.208..865S ]
Weiler, E.J. and Stencel, R.E., 1979. Astron. J., 84, 1372. [BIBCODE 1979AJ.....84.1372W ]