Optical observations of GRB afterglows: GRB 970508 and GRB 980326 revisited

Since January 1997, we have monitored 15 GRB fields, detecting 6 optical/IR afterglows. We have revisited GRB 970508 and GRB 980326. For GRB 970508, we derive a power-law decay exponent alpha =-1.19 (R-band). The luminosity of the host galaxy L relative to the characteristic luminosity L(star ) is in the range 0.06-0.15, i.e. a dwarf galaxy. For GRB 980326, we derive a power-law decay exponent alpha =-1.7, taking into account the new upper limit for the host as R 〉= 27.3 provided by \cite[Bloom & Kulkarni (1998)]{blo98}. This implies one of the fastest GRB optical decays ever measured. The fact that only about 50% of optical transients have been found within the gamma /X-ray error boxes, suggest that either considerable intrinsic absorption is present or that some optical transients display a very fast decline. We also propose that the ``secondary maximum' detected on 17 Apr. 1998 could be explained in the context of the ``SN-like' light curves 2-3 weeks after the GRB, as recently suggested by \cite[Woosley (1999)]{woo99}. Based on observations collected at the German-Spanish Astronomical Centre, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy.}\fnmsep\thanks{Based on observations carried out at the 4.2-m William Herschel Telescope operated by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.