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Early optical afterglows have been observed from GRB 990123, GRB 021004, andGRB 021211, which reveal rich emission features attributed to reverse shocks.It is expected that Swift will discover many more early afterglows. Here weinvestigate in a unified manner both the forward and the reverse external shockemission components, and introduce a straightforward recipe for directlyconstraining the initial Lorentz factor of the fireball using early opticalafterglow data. The scheme is largely independent of the shock microphysics. Weidentify two types of combinations of the reverse and forward shock emission,and explore their parameter regimes. We also discuss a possible diagnostic formagnetized ejecta. There is evidence that the central engine of GRB 990123 isstrongly magnetized.Comment: emulateapj style, 6 pages, 1 figure. Expanded version accepted for  publication in ApJ Part

Gamma‐Ray Burst Early Optical Afterglows: Implications for the Initial Lorentz Factor and the Central Engine