COMMISSION 27 OF THE I.A.U.
                 INFORMATION BULLETIN ON VARIABLE STARS
                             Number 3284

                                                         Konkoly Observatory
                                                         Budapest
                                                         12 January 1989

                                                         HU ISSN 0374 - 0676


              THE SPECTRUM VARIABLE HR 7817=HD 194783

This star has been classified as a B9 Mercury Manganese star by Anderson
and Nordstrom (1977) who also remarked that this object is a probable
spectrum variable. We have begun an observational programme to monitor peculiar 
stars that are most likely to be spectrum variables. The choice of the
stars is basically dependent on the remarks for these stars given in the
Bright Star Catalogue (Hoffleit and Jaschek 1982).
   During the period 28 October to 2 November 1988, five spectra of HR7817
at a dispersion of 31 A per millimeter were obtained with the Zeiss spectrograph
at the Cassegrain focus of the one meter telescope of the Vainu Bappu
Observatory at Kavalur. Visual inspection of the spectra indicated that
Mg II 4481, Si II 4128-31, Mn II 4137, Hg II 3984 and Ca II 3933 are all
variable.
   Figure 1, shows the density tracings near K, Hg II, Si II and Mn II. The
intensities of all these lines were estimated and they indicate a period of
about six days for the variability. There is a possible phase shift between
Mg II variation and the variation due to other lines. The Mg II 4481 line
on 28 October is observed to have a sharp red edge; a suggestion of faint
emission in the red wing. The relative strengths of 4128 and 4131 are also
found to vary. This Mercury-Manganese star has extremely weak Mg, Mn lines
at some phases! Variable faint features at 4077 A and 4206 A are also noticed.
   The lines are all sharp and the projected rotational velocity v sin i is
not likely to exceed 30km/sec. This is consistent with a rotational period
of about six days observed for this star. Periods less than a day can be
ruled out since we should then expect the lines to be much broader than
what is observed. If the elements are concentrated in spots and the period
of variation is that due to rotation, then one should expect in analogy with
other spectrum variables, that HR 7817 is likely to show a measurable magnetic
field even though it is generally believed that Hg Mn stars are non-magnetic.


                           [FIGURE 1]


We are thankful to Shri V. Moorthy for his help at the telescope and 
Shri. A. Ramaswamy for his help at the PDS machine.


                  R. RAJAMOHAN and A. PARANJPYE

                Indian Institute of Astrophysics,
                      Bangalore - 560 034,
                             India



References:

Anderson, J., and Nordstrom, B., 1977, Astron. Astrophys. Suppl. 29, 309. [BIBCODE 1977A&AS...29..309A ]
Hoffleit, D., and Jaschek, C., 1982, The Bright Star Catalogue, 4th revised
   edition, Yale University Observatory, Connecticut, U.S.A. [BIBCODE 1982bscf.book.....H ]