STRING-LOOP CORRECTIONS TO EFFECTIVE ACTION AND BLACK-HOLE INSTABILITIES

Following the ideas of Ref. 3, to account for instability of 2-D black hole solution, we discuss possible imaginary string-loop corrections to the coefficient at the Einstein term in string effective action (EA ). In closed bosonic string theory, such corrections appear because of the tachyon contribution to the integration measure over the moduli. In superstring theory, several possible sources of complex one-string- loop contributions to the EA are considered. It is argued that none of them can account for complex corrections to the Einstein term in the effective action.