POWER-LAW DECAYS IN THE OPTICAL COUNTERPARTS OF GRB-970228 AND GRB-970508

We report on R-C- and K-band observations of the optical counterpart t o GRB 970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2 m t elescope. Eleven R-C-band observations were obtained on 1997 May 12, a nd three on May 14. The counterpart clearly faded between the two nigh ts. On May 12 there was no evidence for variability (<9%) on 10-70 min ute timescales based on 11 R-c-band observations. On May 19 a 1 hr obs ervation set a limit on the K magnitude of K > 18.6. Comparison of the se data points with those obtained by other authors shows that the dec ay of the optical counterpart can be well fitted by a power law of the form f similar to t(-alpha), where alpha = 1.22 +/- 0.03 with occasio nal fluctuations superposed. We note that the decay of the optical cou nterpart to another burst, GRB 970228, can also be well fitted with a power law with exponent alpha = 1.0(-0.5)(+0.2) with occasional fluctu ations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmic-fireball models.