In this paper we present results for all variables identified in
observed fields with the exception of RR Lyr stars.
RR Lyr stars in 
Cen shall be subject of a separate contribution.
We identified a total of 32 periodic variables of which 27 are new discoveries.
The rectangular and equatorial coordinates of these stars
are listed in Table 4 (click here).
The rectangular coordinates correspond to
positions of variables on the V-band "template" images which were
submitted to the editors of A&A (see Appendix A). These images
allow easy identification of all objects listed in Table 4 (click here).
The name of field in which a given variable can be
identified is given in the 6th column.
All frames collected by the OGLE team were deposited at the NASA NSS
Data Center
.
Frames mr10525, mr10524 and mr11042 were used as templates for
fields 5139D, 5139E and 5139F, respectively.
The transformation from rectangular to equatorial
coordinates was derived based on positions of stars which could be
matched with objects from the Guide Star Catalogue (Lasker et al.
1988). We identified 77, 106 and 82 GSC stars
for fields 5139D, 5139E and 5139F, respectively. The adopted frame
solutions reproduce equatorial coordinates of GSC stars with residuals
rarely exceeding 0.5 arcsec.
Table 4:
Rectangular and equatorial coordinates for variables
identified in 
Cen. The X and Y coordinates give
positions of variables on the template images (see text for details).
The last column gives
alternative names for two variables which were previously known.
Variability of OGLEGC17 and OGLEGC20 was already reported in Paper I
Our sample of variables consist of 10 SX Phe stars and 19
certain or probable eclipsing binaries.
Objects OGLEGC17 and OGLEGC20 were already reported in Paper I.
These two stars are located in the overlapping areas of fields 5139A/E
and 5139C/F. Variables 
, 
and
Martin 78 were known before our survey (Jorgensen & Hansen 1984;
Martin 1938).
Table 5 (click here) lists basic characteristics of the light curves of 10 SX Phe star identified in fields 5139D-F. The mean V magnitudes were calculated by numerically integrating the phased light curves after converting them into intensity scale. Photometric data for the remaining variables are given in Table 6 (click here). To determine the most probable periods we used an aov statistic (Schwarzenberg-Czerny 1989, 1991). This statistic allows - in particular - reliable determination of periods for variables with non-sinusoidal light curves (eg. eclipsing variables of EA-type). Phased light curves of SX Phe stars are shown in Fig. 2 (click here) while Figs. 3 (click here) and 4 (click here) present light curves for the remaining variables.
Figure 5 (click here) shows location of variables with known color on the
cluster color-magnitude diagram (CMD).
For the SX Phe stars marked positions correspond to the intensity-averaged
magnitudes. For the remaining variables we marked positions corresponding
to magnitude at maximum light. The main sequence and red giant branch
are marked with
double lines in Fig. 5 (click here). The large width of these sequences is due
to the well known spread of metallicity exhibited by stars in 
Cen
(Woolley 1966).
Table 5:
Light-curve parameters for SX Phe stars from fields D-F in 
Cen.
is the range of observed variations in the V band.
(V-I) is the observed color at the maximum light.
The period is given in days
All SX Phe stars as well as some eclipsing binaries are located on the cluster CMD among candidate blue stragglers. Four eclipsing binaries occupy positions at the base of the subgiant branch. Among them there are two systems with EA-type light curves: OGLEGC17 and OGLEGC51. The first system has been already reported in Paper I. The second one is a new discovery. The orbital period of OGLEGC51, P=4.64 day, connected with the shape of its light curve indicate that this binary is a detached system. The phase coverage at the primary minimum is poor and observed depth of the primary eclipse should be treated as a lower limit only. The variable was caught twice in the primary eclipse and twice in the secondary eclipse. This allowed to determine the period of OGLEGC51 with confidence.
Figure 4: Phased V light curves for non-pulsating variables
identified in fields 5139D-F. Inserted labels give names of variables
and their periods
Figure 4:
Phased V light curves for non-pulsating variables
identified in fields 5139D-F. Inserted labels give names of variables
and their periods
Figure 5: The schematic CMD for 
Cen with positions
of variables from fields D-F marked. The triangles represent EA-type
eclipsing binaries, the squares EW/EB-type binaries, the asterisks
SX Phe stars and the open circles probable ellipsoidal or
spotted variables
Five variables occupy positions on the upper part of the main-sequence of the cluster. OGLEGC53 exhibit EB-type light curve and has period close to 0.50 day. It may be either a semidetached or nearly contact binary. OGLEGC49, OGLEGC54 and OGLEGC56 show light curves and periods typical for W UMa-type contact binaries. OGLEGC43 also shows EW-type light curve but at the same time its period, P=1.10 day, is unexpectedly long for a contact binary of late spectral type. The light curve of this star shows two distinct minima. Hence, we can rule out a possibility that OGLEGC43 is a pulsating variable with P=0.55 day located in the galactic halo behind the cluster.
Another interesting binary from our list is OGLEGC52. This star is located slightly above the horizontal branch of the cluster near the blue edge of the instability strip. Its light curve suggests a semi-detached configuration with a red component filling its Roche lobe. The secondary eclipse is shallow but the depth of the primary eclipse indicates comparable sizes of components of the system. OGLEGC52 may be composed of a horizontal branch star and a red sub-giant. On the other hand this star can be a field binary located in the cluster foreground. The out-of-eclipse part of the light curve does not show presence of any secondary periodicity. Hence, the blue component of OGLEGC52 is not a RR Lyr star. Some scatter visible in the light curve before primary eclipse is due to problems with photometring over-exposed images of the star.
We consider some of our period determinations, especially those for OGLEGC61 and OGLEGC65, as preliminary. For OGLEGC61 we obtained reasonable, low-amplitude light curves for two values of the orbital period: 0.631 and 0.479 day. For OGLEGC65 the possible periods are 0.512 and 0.408 day.
The periods of all identified SX Phe stars are rather firmly established. For variables of this type we calculated the power spectra using the program based on a CLEAN algorithm (Roberts et al. 1987). For all stars the derived power spectra show a very prominent peak at positions corresponding to the periods listed in Table 5 (click here).
The variable OGLEGC41 is located about 0.3 mag to the red of the subgiant branch of the cluster. We adopted for this star a period P=1.62 day but it cannot be ruled out that the real period is twice shorter. For P=1.62 day the likely reason of observed variability would be binary nature of OGLEGC41. This star could be either an eclipsing binary or an ellipsoidal variable. For P=0.81 day it could belong to BY Dra or FK Com type variables. The relatively red color of OGLEGC41 is consistent with the hypothesis that it belongs to spotted variables.
The contact binary OGLEGC67 is located more than 0.5 mag to the red of the base of the subgiant branch on the cluster CMD. It is most probably a foreground star.
The low amplitude variable OGLEGC68 is located about 2 mag above cluster turnoff and 0.3 mag to the blue of the subgiant branch. We adopted for it a period of 0.65 day but the light curve can be phased also with a period of 1.3 day. For P=0.65 day the star could be a foreground spotted variable. For P=1.3 day it could be an ellipsoidal variable composed of subgiant and a star from an extended horizontal branch.
The 
Cen cluster is located at an intermediate galactic
latitude of b=+15 deg. Therefore, one cannot assume that
all variables listed in Table 4 (click here) are cluster members.
Figure 6 (click here) shows the period versus absolute magnitude diagram for
10 SX Phe stars listed in Table 5 (click here). The standard relations for the
F-mode pulsators (solid line) and the H-mode pulsators (dashed line) and for
(upper line) and 
(lower line)
are also shown. The calibration 
was taken from
Nemec et al. (1994). We adopted for the cluster an apparent
distance modulus 
while calculating absolute
magnitudes of SX Phe stars. The assumed range of metallicities is based
on results published by Brown & Wallerstein (1993) and
Vanture
et al. (1994). The observed luminosities of reported SX Phe
stars are consistent with the hypothesis that all of them are
members of the cluster.
We have applied the absolute brightness calibration established by
Rucinski (1995) to calculate 
for newly discovered
contact binaries. Rucinski's calibration gives 
as a function
of period, unreddened color 
and metallicity:
We adopted for all systems 
what is the mean metallicity for the cluster stars.
Figure 7 (click here) shows the period versus an apparent distance modulus
diagram for contact binaries identified in fields 5139D-F
of 
Cen. Specifically the following systems listed in
Table 6 (click here) were considered to be "normal" W UMa-type binaries:
OGLEGC20, 44, 48, 49, 54, 56, 57, 58, 61, 64, 65 and 67.
An apparent distance modulus was calculated for each
system as a difference between its V magnitude and 
.
An apparent distance modulus for 
Cen is estimated at
(Nemec et al. 1994). The systems with mostly
deviating values of 
are OGLEGC48, OGLEGC57
and OGLEGC67 (this system is not marked in Fig. 7 (click here)).
These binaries are probably
a foreground variables. The remaining systems plotted in Fig. 7 (click here)
are likely members of the cluster.
Table 6:
Light-curve parameters for eclipsing binaries discovered in fields
D-F in 
Cen.
(V-I) is the observed color at the maximum light.
is the time of minimum light
Clearly some additional data are needed to establish with more confidence
which variables identified in our survey are members of 
Cen.
We note that potential field interlopers should exhibit radial velocity
differing significantly from the velocity observed for the cluster.
Radial velocity of 
Cen is 
(e.g. Zinn
1985) which is a much higher value than velocities observed for
field stars from the cluster region.
Figure 6:
Period vs. absolute magnitude diagram for SX Phe stars from the
field of 
Cen
Figure 7: Period vs. apparent distance modulus diagram for contact
binaries from the field of 
Cen. A horizontal line at
corresponds to distance modulus of the cluster.
Error bars correspond to the formal uncertainty of absolute magnitudes
derived using Rucinski's (1995) calibration
