The Strömgren uvby system was used in all photometric measurements.
During 60 nights (9 nights from Mar. 15 to 29, 1987, 24 from Feb. 30 to
Mar. 31, 1988, 14 from Apr. 1 to May 8, 1989, 3 from May 6 to 16, 1990 and
10 from Apr. 4 to 30, 1991) V906 Sco was observed at ESO (La Silla,
Chile), with the 50 cm Strömgren Automatic Telescope (SAT) equipped
with the six-channel spectrograph-photometer (four channels for
simultaneous uvby photometry and two channels for simultaneous
measurements of 
narrow and wide filters) and photon
counting system described by Nielsen et al. (1987). In the measurements a
circular diaphram of 
diameter was used. Apart
from the
third component of this triple system, detected spectroscopically,
no other stars were detected inside this diaphragm, despite the
richness of the field close to V906 Sco (which is approximately in the
central projected region of M7). The system has been reported (Van den
Bos 1931) to constitute a visual pair; this could not be discerned
during our observations, due to the close proximity of V906 Sco and
its companion.
Extinction corrections were based on the nightly coefficients from the three comparison stars and other constant stars. When needed, linear or quadratic corrections were applied for correcting for eventual instrumental drifts (uncooled photomultipliers), and/or for variations in the atmosphere transparency during the night. The dead times of the six EMI 9789QA, S11, selected for low dark counts, were accounted for in the reduction procedure.
HD 
, HD 
(HR 
) and HD 
(HR 
) were used as comparison stars and observed alternately
between the measurements of V906 Sco. The first two stars are located
very close to V906 Sco (maximum projected distance in the sky of 
) while the last one is a bit more distant (
) from the
system. All three stars were found to be constant within the
observational accuracy throughout the observing periods. The
observations of HD and HD were transformed to
C1 (HD ) by means of the constant difference of
magnitude between them and C1 obtained from all nights. For
the sake of completeness we repeat here Table 1 (click here) from
Alencar et al.
(1997), with information for V906 Sco and comparison stars.
Typical rms errors of one magnitude difference between the comparison
stars in the measurements were found to be: 
for 
, 
for 
and 
). Most
of the phases were covered at least twice. The observations are
available in electronic form at the CDS, as Table 2, containing 1117
magnitude differences V906 Sco - HD in the instrumental
system. Figure 1 (click here) shows the observed light curves (y, b-y and u-b),
together with the theoretical solution obtained by Alencar et al.
(1997).
The coefficients for transformation to the standard uvby system obtained by us agree very well with those determined by Olsen (1993), which are more accurate and based on observations from 1985, 1986 and 1987. Those coefficients should, then, be adopted. They are repeated in Table 3 (click here) for convenient use, together with the transformation equations.
Table 3: Transformation coefficients from Olsen(1993); 
indicates a difference
in the instrumental system (directly from Table 2) and
a transformed value in the standard system
Further discussion of these observations, including a study of
times of minima (which makes evident the existence of apsidal
motion), some observations in H
, and a spectroscopic study
based on new spectroscopic observations will
be published as part of a detailed photometric analysis of V906 Sco,
based on these uvby light curves and index measurements
(Alencar et al. 1997).
Acknowledgements
This investigation was supported by allocation of ESO observing time and by grants from the Danish Natural Science Research Council, The Danish Board for Astronomical Research, and from the Brazilian institutions CNPq, FAPEMIG, FINEP, CAPES. Support was also received through the programme of cultural, educational, and scientific cooperation between Spain and Denmark. This research has made use of the Simbad database, operated at CDS, Strasbourg, France.
