COMMISSION 27 OF THE I. A. U.
           INFORMATION BULLETIN ON VARIABLE STARS
                        Number 3655

                                              Konkoly Observatory
                                              Budapest
                                              29 August 1991

                                              HU ISSN 0374 - 0676


     The photometric variability of the B9p star HD 137509


Cowley & Houk (1975) first pointed out the interesting nature of the B9pSiCrFe
star HD 137509 (= CPD -70d 2069 = SAO 257290) which exhibits very strong Cr
II, Fe II and Ti II lines in its spectrum. This star was introduced in
photometric programs, but it was only recently that its variability was
reported on the basis of 10 UBV measurements (Loden & Sundman 1989).
Simultaneously Mathys (1991) discovered a strong reversing longitudinal
magnetic field, and he noted that the spectrum of HD 137509 undergoes fast and
spectacular variations. Most magnetic Bp stars are known to be photometric
variables, but the spectropolarimetric observations reported by Mathys (1991)
revives our interest for this star. This star should indeed have quite large
surface chemical inhomogeneities, and furthermore its two magnetic poles are
visible during a whole rotation cycle, which is the most favourable
circumstance for surface mapping. Before any further study of the spectrum
variations, we need to determine precisely the rotation period. This could be
done most efficiently with new photometric observations. At the moment the 9
measurements of the longitudinal magnetic field recorded by Mathys are too few
to derive safely a period, but they will be quite useful to avoid spurious
periods or to remove ambiguities between different values coming out from the
photometry alone.

In March and April 1991, we got 36 new measurements of HD 137509 in the Geneva
seven-color photometric system with the photometer P7 attached to the 0.7 m
Swiss telescope located at La Silla (Chile). They were reduced in the general
reduction frame at Geneva Observatory. The accuracy of the data for several
nights appears to be somewhat lower due to poor weather and to the very
southern declination of the star. Accordingly low weights were assigned to
these measurements. We discarded for further analysis only the measurements
with a null weight (7 measurements).


                                   [FIGURE 1]

 Figure 1: Photometric and longitudinal magnetic field variations of HD 
 137509. The phase origin is HJD 2448344.575. Open squares are data 
 excluded from the period analysis and from the least-squares fits (low 
 weight photometric data or polarimetric data of Bohlender & Landstreet).


HD 137509 undergoes quite large variations for a magnetic Bp star, with Delta U
~= 0.1 mag and Delta V = 0.05 mag. The phase dispersion method (Stellingwerf
1978) and Renson's (1978) theta_1 test were applied separately to U, B and V
magnitudes to determine the period P. Then least-squares fits of the
observations were computed with a modified Newton method, assuming that the
model function is a sin e wave and its first harmonic. The ultimate accuracy
on the period P was achieved by slightly varying P and looking for the minimum
of the least-squares fit standard deviation for the three magnitudes. The best
period coming out from the photometry is P = 4.5568 days, but this value is not
supported by the magnetic data. Other possible values of the period from the
photometry alone are 4.491, 4.505 and 4.594 days. The only one that
satisfactorily represents the magnetic variations is:

                P = 4.4912 +-0.0010 days

which is adopted as the rotation period of HD 137509. A minimum of U occurred
at HJD 2448344.575. Two more polarimetric measurements (Bohlender &
Landstreet, private communication) support our period, as well as data on the
crossover effect derived from the spectropolarimetry (Mathys, in preparation).
The figure shows the photometric and the magnetic variations, and the
least-squares fits with the adopted period.


     T. LANZ                      G. MATHYS
     Institut d'Astronomie        Observatoire de Geneve
     Universite de Lausanne       CH-1290 Sauverny
     CH-1290 Chavannes-des-Bois   Switzerland
     Switzerland

References

Cowley, A.P., Houk, N.: 1975, PASP 87, 527 [BIBCODE 1975PASP...87..527C ]
Loden, L.O., Sundman, A.: 1989, J. Astrophys. Astron. 10, 183 [BIBCODE 1989JApA...10..183L ]
Mathys, G.: 1991, A&AS 89, 121 [BIBCODE 1991A&AS...89..121M ]
Renson, P.: 1978, A&A 63, 125 [BIBCODE 1978A&A....63..125R ]
Stellingwerf, R.F.: 1978, ApJ 224, 953 [BIBCODE 1978ApJ...224..953S ]