Bulge RR Lyrae stars in the VVV tile b201

Context. The VISTA Variables in the Via Lactea (VVV) Survey is one of the six ESO public surveys currently ongoing at the VISTA telescope on Cerro Paranal, Chile. VVV uses near-IR (ZYJHK(s)) filters that at present provide photometry to a depth of K-s similar to 17.0 mag in up to 36 epochs spanning over four years, and aim at discovering more than 106 variable sources as well as trace the structure of the Galactic bulge and part of the southern disk. Aims. A variability search was performed to find RR Lyrae variable stars. The low stellar density of the VVV tile b201, which is centered at (l, b) similar to (-9 degrees,-9 degrees), makes it suitable to search for variable stars. Previous studies have identified some RR Lyrae stars using optical bands that served to test our search procedure. The main goal is to measure the reddening, interstellar extinction, and distances of the RR Lyrae stars and to study their distribution on the Milky Way bulge. Methods. For each star in the tile with more than 25 epochs (similar to 90% of the objects down to K-s 17.0 mag), the standard deviation and chi(2) test were calculated to identify variable candidates. Periods were determined using the analysis of variance. Objects with periods in the RR Lyrae range of 0.2 <= P <= 1.2 days were selected as candidate RR Lyrae. They were individually examined to exclude false positives. Results. A total of 1.5 sq deg were analyzed, and we found 39 RR Lyr stars, 27 of which belong to the ab-type and 12 to the c-type. Our analysis recovers all the previously identified RR Lyrae variables in the field and discovers 29 new RR Lyr stars. The reddening and extinction toward all the RRab stars in this tile were derived, and distance estimations were obtained through the period-luminosity relation. Despite the limited amount of RR Lyrae stars studied, our results are consistent with a spheroidal or central distribution around similar to 8.1 and similar to 8.5 kpc. for either the Cardelli or Nishiyama extinction law. Our analysis does not reveal a stream-like structure. Nevertheless, a larger area must be analyzed to definitively rule out streams.