Variable stars in the fornax dsph galaxy. I. The globular cluster fornax

Variable stars have been identified for the first time in Fornax 4, the globular cluster located near the center of the Fornax dwarf spheroidal galaxy. By applying the image-subtraction technique to B and V time series photometry obtained with the MagIC camera of the 6.5m Magellan Clay telescope and with the wide field imager of the 4 m Blanco CTIO telescope, we detected 27 RR Lyrae stars (22 fundamental-mode, 3 first-overtone, and 2 double-mode pulsators) in a 2.4' x 2.4' area centered on Fornax 4. The average and minimum periods of the ab-type RR Lyrae stars, < Pab > = 0: 594 days and P(ab,min) = 0.5191 days, respectively, as well as the revised position of the cluster in the horizontal branch type-metallicity plane, all consistently point to an Oosterhoff-intermediate status for the cluster, unlike what is seen for the vast majority of Galactic globular clusters, but in agreement with previous indications for the other globular clusters in Fornax. The average apparent magnitude of the RR Lyrae stars located within 3000 from the cluster center is < V(RR)> = 21.43 +/- 0.03 mag (sigma = 0.10 mag, average of 12 stars), leading to a true distance modulus of mu(0) = 20.64 +/- 0.09 mag or mu(0) = 20.53 +/- 0.09 mag, depending on whether a low ([Fe/H] = -2.0) or a moderately high ([Fe/H] = -1: 5) metallicity is adopted.