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ngc  4833 is a southern globular cluster lying near the galactic plane in the constellation musca .
its variable stars have not been studied in detail for nearly 35 years since observations by demers & wehlau ( 1977 ) .
the first survey of variable stars in the cluster s vicinity was done by bailey ( 1924 ) who found two variables , both of which are likely not cluster members . later studies revealed a total of 16 variables in the vicinity of the cluster ( wright 1941 ; sawyer - hogg 1973 ) . among these
were the two variables found by bailey with most of the remaining being cluster variables , known today as rr lyrae variables .
using photographic plates demers & wehlau ( 1977 ) did b and v photometry of the cluster identifying 24 variables and determined accurate periods for 15 rr lyrae variables .
they concluded that the cluster was oosterhoff type ii ( oosterhoff 1939 ) .
the most recent study was melbourne et al .
they investigated extinction near ngc  4833 and determined the positions of the rr lyrae variables on a color - magnitude diagram but did not do time - series photometry .
ngc  4833 is 6.6 kpc from the sun , has a core radius of 1.0 , and a half light radius of 2.41 which make it a relatively easy target for searching for and investigating its variable stars ( harris 1996 ) .
given the lack of recent detailed study of the variables in ngc  4833 we added the cluster to our continuing program to study in detail a number of southern globular star clusters ( conroy et al .
2012 , toddy et al .
2012 ) . with the combination of ccd images , an observation window spanning more than 4 months , and an image subtraction package developed by alard ( 2000 )
, we are able to detect , analyze variable stars , and produce unprecedented high - quality light curves and periods for each variable . in this study
we present the preliminary results of our observations of ngc  4833 including a color - magnitude diagram , as well as a bailey diagram , complete light curves , and periods for 27 variables , with 10 of them being newly identified .
images of ngc 4833 were obtained during 14 nights between 30 january and 25 june , 2011 , using the southeastern association for research in astronomy ( sara ) 0.6 meter telescope located at cerro tololo interamerican observatory . in all , nearly 1700 images were obtained in the v - band . also , multicolor sequences were done on 9 march , 1 , 3 , and 6 may , 2011 , as a vri sequence on 9 march , and a bvri sequence on 1 , 3 , and 6 may .
an apogee alta e6 camera was used for the observations with a 1024@xmath01024 pixel kodak kaf1001e chip , a gain of 1.5 electron per adu count , and an rms noise of 8.9 electrons .
we used 1@xmath01 binning resulting in an image scale of 0.61/px resulting in a 10@xmath010 arcmin@xmath1 field of view . on all nights of observation
, 120 second exposures were taken using a bessel v filter .
the exposure time allowed us to detect dimmer variables but caused many of the brighter stars in the field to be overexposed .
typical seeing ranged between 1.2 and 1.7 in the v - band .
all images were debiased , flat - fielded , dark subtracted , and cleaned of hot , cold , and bad pixels .
images were then analyzed using the isis2.2 package ( alard & lupton 1998 , alard 2000 ) .
the package works by convolving the reference image shown in figure 1 , composed of a combination of the highest quality images obtained , to the point spread function of each individual image , then subtracting each image from the reference image to determine if any change in intensity has occurred .
a resulting variable image identifies possible variables and is shown in figure 2 .
a more detailed discussion of the method and how we used it is given elsewhere in this volume by toddy et al .
( 2012 ) .    .
note that brightest stars in the image are overexposed . ]
due to the more than 4-month observation span , we were able to determine the periods of most of the variables with periods less than 1 day to an accuracy of @xmath2 days .
those variables exhibiting the blahko effect were less accurate due to modulation of their light curves ( blahko 1907 , smith 2004 ) .
demers & wehlau s ( 1977 ) variables were identified visually and astrometrically and then confirmed by comparing their periods with our own .
several of the variables identified by demers & wehlau ( 1977 ) were outside the field of view of our images thus were not be observed .
these include their variables v1 , v2 , v8 and v10 .
we were not able to produce time - series photometry of two other variables , v9 and v16 .
though they were in our field of view they were overexposed giants .
lrc rrab & 10 & 0.69591 + rrc & 7 & 0.39555 + rre & 2 & 0.30950 + eclipsing & 2 & 0.32431 + sx phe & 6 & 0.05847                table 1 summarizes the results of our study . in total we detected 27 variables with 10 of them being newly identified . of the variables that were in our field of view and listed by demers & welhau ( 1977 ) we were able to identify all but two .
the two that we did not detect happened to be overexposed . of the total we detected 6
were newly discovered sx phoenicis variables , 19 rr lyrae variables , and 2 eclipsing variables .
table 2 gives the properties of each of the variables .
the sx phoenicis variables are characterized by short periods and relatively low amplitudes .
the mean period of the sx phoenicis variables is 0.05847 days , with a minimum of 0.04425 days and a maximum of 0.07191 days along with v amplitudes ranging from 0.2 to 0.7 magnitudes .
these amplitudes are only estimates given that many of the sx phoenicis variables appear to be modulated and blended with other stars .
nv5 was detected by isis only on 5 april .
the others were detected on all of the nights .
all of the sx phoenicis stars were within the half - mass radius ( 2.4 ) of the cluster as listed by harris ( 1996 ) .
the phased light curves for the sx phoenicis stars are shown in figure 3 .        of the rr lyrae variables identified in the cluster 10 were rrab variables , with a mean period of 0.69591 days , 7 were rrc variables , with a mean period of 0.39555 days , and variable v20 which may be a second overtone ( rre ) variable previously noted by demers & wehlau ( 1977 ) . but given v20 s modulated light curve it may be a type rrd double - mode variable ( alcock et al .
1996 , bono et al .
our newly discovered variable stars include a w ursae majoris star ( period of 0.3629 days ) , an rrab star ( period of 0.8740 days ) , another second overtone ( rre ) variable ( period of 0.3179 days ) , and an algol type binary star ( period of 0.7231 days ) .
in total we identified 10 new variables in the cluster .
the phased light curves for these variables are shown in figure 4 .
in addition to the ratio of rrab to rrc lyrae variables the bailey diagram presented in figure 5 shows the cluster is of the oosterhoff type ii cluster . in this diagram
the fitting formulae are from zorotovic et al .
figure 5 also shows the two outlying rre variables , indicated by the open triangles .
color - magnitude diagrams ( cmd ) are useful in determining cluster membership . to verify cluster membership we produced a color - magnitude diagram for ngc 4833 shown in figure 6 with the various types of variable stars indicated . to construct this diagram we combined 165 v band images , with an exposure of 120 seconds , and 175 r band images , with an exposure of 45 seconds .
the images were obtained as part of a bvri color sequence on 9 march , 1 , 3 , and 6 may , 2011 .
daophot was used to determine v and r magnitudes of the stars ( stetson 1987 ) . in the cmd shown in figure
6 rrab and rrc variables are indicated by the red and blue colored points , respectively .
rrc variables tend to lie on the warmer ( left ) side of the instability strip whereas rrab variables are to the cooler ( right ) of the instability strip .
scatter in the variables is partly due to blending of the variables with other stars , our averaging of several hundred images before using daophot , and possible variable reddening in the direction of the cluster .
we assumed fixed atmospheric extinction coefficients for each color and for all nights .
[ cols="^,^,^,^,^,^ " , ]     of the rr lyrae variables only one , v15 , appears to be a blend .
v15 is shown in figure 6 as the hollow red square to the right of the instability strip .
other than this errant variable the remainder appear near the instability strip .
in addition to blending there is significant extinction across the cluster .
the e(b - v ) is listed as 0.32 and melbourne et al .
( 2000 ) found that this varies depending on location .
because of this variable interstellar extinction some scatter is introduced into our mean magnitudes of the rr lyrae variables .
the sx phoenicis variables all lie to the left of the giant branch .
after close inspection of the images all but one of our sx phoenicis variables appeared to be blended with stars of equal or greater brightness . further inspection of hubble space telescope images confirmed this .
variables nv4 and nv5 appear to be blended with giant stars whereas nv7 , nv8 , and nv9 are blended with stars of similar brightness to themselves .
nv6 is the only sx phoenicis variable that does not appear to be blended with a relatively bright or comparable bright star .
for nearly 5 months from late january through june of 2011 we observed the globular cluster ngc  4833 using the sara 0.6 meter telescope at ctio .
using the image subtraction software isis we were able to find precise periods of 17 previously known variables and 10 newly discovered variables stars . of these new discoveries we found 6 sx phoenicis variables .
the other 4 newly discovered variables were 2 eclipsing , one rrab , and one apparent rre . in total , we classified 10 of the variables as type rrab , with a mean period of 0.69591 days , 7 as type rrc with a mean period of 0.39555 days , perhaps 2 lower amplitude type rre , with a mean period of 0.30950 days , and 2 eclipsing binaries , with a mean period of 0.3243 days . both the bailey diagram and ratio of @xmath3 are consistent with the cluster being of type oosterhoff type ii .
we thank c. alard for making isis 2.2 publicly available .
this project was funded in part by the butler institute for research and scholarship .
the authors also thank f. levinson for a generous gift enabling butler university s membership in the sara consortium .
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we report the discovery of 6 sx phoenicis stars in the southern globular cluster ngc 4833 . 
 images were obtained from january through june 2011 with the southeastern association for research in astronomy 0.6 meter telescope located at cerro tololo interamerican observatory . 
 the image subtraction method of alard & lupton ( 1998 ) was used to search for variable stars in the cluster . 
 we confirmed 17 previously cataloged variables by demers & wehlau(1977 ) . 
 in addition to the previously known variables we have identified 10 new variables . of the total number of confirmed variables in our 10@xmath010 arcmin@xmath1 field , we classified 10 rrab variables , with a mean period of 0.69591 days , 7 rrc , with a mean period of 0.39555 days , 2 possible rre variables with a mean period of 0.30950 days , a w ursae majoris contact binary , an algol - type binary , and the 6 sx phoenicis stars with a mean period of 0.05847 days . 
 the periods , relative numbers of rrab and rrc variables , and bailey diagram are indicative of the cluster being of the oosterhoff type ii . 
 we present the phased - light curves , periods of previously known variables and the periods and classifications of the newly discovered variables , and their location on the color - magnitude diagram .