COMMISSION 27 OF THE I. A. U.
         INFORMATION BULLETIN ON VARIABLE STARS
                   Number 3199

                                 Konkoly Observatory
                                 Budapest
                                 6 June 1988

                                 HU ISSN 0374-0676


         THE PERIOD OF THE A0p Si PROBABLE
             BLUE STRAGGLER NGC 6281-9

    The star HD 153947=DM-37 11215=NGC 6281-9 is listed
as a probable blue straggler in the cluster NGC 6281 by
Mermilliod (1982). Like several other blue stragglers,
it is classified as A0p Si, so that it could be considered 
as potentially variable, like all magnetic peculiar
stars. It was included in our programme of determination
of periods of Ap stars in clusters, although its membership 
is not firmly established: proper motions and
radial velocity are still lacking, and the star lies at
1.5 cluster radii from the cluster centre, in projection
on the sky. Pending confirmation of its membership of
NGC 6281, this star may be considered a most interesting
object.

    The first measurements, made in 1987 at La Silla
with the 70-cm Swiss telescope, in the seven filters of
the Geneva photometric system, show a large amplitude but
were insufficient to disentangle the two possible periods
p=2.7 or P=0.73 days. More measurements have been made
from La Silla in May 1988, which clearly show that the
longer period should be preferred. The most probable


                      [FIGURE 1]

 Figure 1. Lightcurves of the star NGC 6281-9 in three
           Geneva passbands


period is thus

P = 2.662 +/- 0.003 days

although some other nearby periods may be possible too,
such as P=2.640 or 2.686, the latter being less 
satisfactory. This ambiguity is due to the almost one-year
gap between the observations of 1987 and those of 1988.
It should be suppressed by future measurements later in
the season.

    Fig. 1 shows the lightcurves of NGC 6281-9 in the
three Geneva bandpasses [U], [B] and V. The zero phase
corresponds to the first 1987 observation, i.e. HJD
2446965.645. We see that the amplitude increases from
about 0.07m in V to 0.12 in [U], which is rather typical
of Si stars. The period, too, is rather typical of old
Si stars (NGC 6281 has a log (age) =/~ 8.35): it is only
slightly greater than the value (about 1.8) corresponding
to the maximum of the period distribution of these stars
(North, 1987).




P. NORTH
Institut d'astronomie
de l'Universite de
Lausanne
CH-1290 CHAVANNES-DES-BOIS
Switzerland

G. JASNIEWICZ
Observatoire de Strasbourg
11, rue de l'Universite
F-67000 STRASBOURG
France


Ch. WAELKENS
Astronomisch Instituut
Katholieke Universiteit Leuven
Celestijnenlaan 200B
B-3030 HEVERLEE
Belgium


References:

Mermilliod, J.-C.: 1982, Astron. Astrophys. 109, 37 [BIBCODE 1982A&A...109...37M ]
North, P.: 1987, Astron. Astrophys. Suppl. Ser. 69, 371 [BIBCODE 1987A&AS...69..371N ]