OWENS VALLEY INTERFEROMETRIC OBSERVATIONS OF THE GAMMA-RAY BURSTS GRB-970228 AND GRB-970508

We have obtained new observations of the gamma-ray bursts GRB 970228 a nd GRB 970508 with the Owens Valley Radio Observatory. GRB 970228 was observed in the 1 and 3 mm continuum bands. The held was centered on t he position of the optical counterpart approximately 17 days after the initial GRB burst. GRB 970228 was not detected at either wavelength, which places 3 sigma upper limits of 2.16 mJy at 92.7 GHz and 38.1 mJy at 242.4 GHz. GRB 970508 was observed in the 3 mm continuum band, cen tered first on the position of the X-ray source SAX J0653.8+7916 and t hen on the position of the optical counterpart and the transient sourc e detected by the Very Large Array. A series of five observations were made from 1997 May 10 (similar to 2 days after initial burst detectio n) to 1997 May 16 (similar to 8 days after burst). GRB 970508 was not detected during any observation. We combine data from May 12 to May 16 to place a solid 3 sigma upper limit of 3.0 mJy at 86.8 GHz. Assuming the gamma-ray burst can be modeled by a cosmological fireball, our no ndetection supports the interpretation by Frail and coworkers that lar ge fluctuations seen in the centimeter-wave radio fluxes during this t ime are not due to changes in the source itself but are instead due to interstellar scintillation when the early fireball is nearly a point source.