Figure 6 (click here) shows a CMD of NGC 6397 with marked positions of 8 out of 9
variables detected in our survey. Positions of pulsating
variables correspond to their average magnitudes and colors.
For RR Lyr variable V3 we obtained intensity averaged magnitudes 
and 
.
This star occupies a position about 3 magnitudes below the horizontal
branch of NGC 6397 and it is clearly a background object.
For SX Phe stars we approximated average magnitudes
with the arithmetic mean of 
and 
. We
obtained 
and 
for V10 and V11, respectively.
Both SX Phe stars detected in our survey are located
among candidate blue stragglers on the cluster CMD.
Let us assume for a moment that V10 and V11 are members of the cluster.
Adopting for NGC 6397 
(Djorgovski 1993) we obtain 
and
for the observed absolute magnitudes of V10 and V11,
respectively.
Figure 4: Phased V-band light curves of variables V3-4, V7-9 and V11
Figure 5: Light curves of variables V5, V6 and V10 obtained on
the night of July 8/9, 1995
On the other hand, we may use the period-luminosity
relation to estimate absolute magnitudes of SX Phe stars. According to
criteria introduced by McNamara (1995) V10 and V11 are the
first-overtone pulsators. We adopted the relation 
(McNamara 1995) and obtained 
and 
for V10 and V11, respectively. A very good
agreement between the observed and calculated values of 
indicates
that both SX Phe stars are likely members of the cluster.
We discovered one probable and four certain contact binaries in
the cluster field.
Systems V4 and V9 are located about 0.3 mag to the red of the
cluster main sequence.
Binary V7 is located close to the cluster turnoff among candidates for
faint blue stragglers while V8 occupies
a position only slightly above the main sequence.
We have applied the absolute brightness calibration established by
Rucinski (1995) to calculate 
for newly
discovered contact binaries.
Assuming that all systems are members of the cluster we
adopted for them 
and E(B-V)=0.18.
Values of 
were derived by
adopting again 
for all systems. The derived values of 
and 
are given in Table 4 (click here). The presented results, if taken strictly,
indicate that
V4 and V9 are background objects while V7 and V8 are located
in the cluster foreground. On the other hand, the differences between
and 
are relatively small.
The Rucinski's (1995) calibration is rather preliminary
with respect to metal poor systems and the calibration stars
exhibit a scatter with 
mag around
the adopted relation.
Therefore, the available data do not allow to reject
a hypothesis about cluster membership of any of newly
discovered contact systems.
We excluded from the above discussion the variable V6 for which only a rough estimate of a period is accessible for a moment. This star is located on the CMD far away to the red from the cluster main-sequence. Most probably V6 is not a member of NGC 6397.
We have failed to determine the B-V color for variable V5. Therefore its position on the cluster CMD is unknown. This star was positioned very closely to the edge of the monitored field. Its photometry was difficult also because of the presence of 2 close and bright companions. A visual inspection of several frames confirmed beyond any doubts a variability of V5. The light curve of this star shows two distinct minima. Because of the noisy photometry it is difficult to prove or disprove a hypothesis that V5 is a contact binary. We may only note that almost all known eclipsing main-sequence binaries with periods shorter than about 0.4 day show EW-type light curves. The light curve of V5 is very atypical in this respect. Further observations are needed to clarify status of V5.
Figure 6: The color-magnitude diagram
of NGC 6397 with the positions of the variables.
Squares correspond to certain or probable (e.g. V6) eclipsing binaries,
filled triangles to SX Phe stars and an open triangle to RR Lyr star V3.
For the sake of clarity, stars from the most crowded part of
the observed field are not marked
Table 4: Calculated and observed absolute magnitudes of contact binaries from
the field of NGC 6397. Both quantities were derived under the assumed
cluster membership of variables