  Optical Transients from gamma-ray burst sources, in addition to offering a
distance determination, convey important information on the physics of the
emission mechanism, and perhaps also about the underlying energy source. As the
gamma-ray phenomenon is extremely diverse, with time scales spanning several
orders of magnitude, some diversity in optical counterpart signatures appears
plausible. We have studied the Optical Transient, which accompanied the
gamma-ray burst of May 8, 1997 (GRB 970508). Observations conducted at the
2.5-m Nordic Optical Telescope (NOT) and the 2.2-m telescope at the
German-Spanish Calar Alto observatory (CAHA) cover the time interval starting 3
hours 5 minutes to 96 days after the high energy event. This brackets all other
published observations, including radio. When analyzed in conjunction with
optical data from other observatories, evidence emerges for a composite light
curve. The first interval, from 3 to 8 hours after the event was characterized
by a constant, or slowly declining brightness. At a later moment the brightness
started increasing rapidly, and reached a maximum approximately 40 hours after
the GRB. From that moment the GRB brightness decayed approximately as a
power-law of index -1.21. The last observation, after 96 days, m_R =
24.28+-0.10, is brighter than the extrapolated power-law, and hints that a
constant component, m_R = 25.50+-0.40 is present. The OT is unresolved (FWHM
0.83") at the faintest magnitude level. The brightness of the optical
transient, its duration and the general shape of the light curve sets this
source apart from the single other optical transient known, that of the
February 28, 1997 event.
