Stringy probe particle and force balance - art. no. 104046

We directly derive the classical equation of motion, which governs the cent er of mass of a test string, from the string action. In a certain case, the equation is basically the same as the one derived by Papapetrou, Dixon, an d Wald for a test extended body. We also discuss the force balance using a stringy probe particle for an exact spinning multisoliton solution of Einst ein-Maxwell-dilaton-axion theory. It is well known that the force balance c ondition yields the saturation of the Bogomol'nyi type bound in the lowest order. In the present formulation the gyromagnetic ratio of the stringy pro be particle is automatically determined to be g = 2, which is the same valu e as the background soliton. As a result we can confirm the force balance v ia the gravitational spin-spin interaction. [S0556-2821(99)05620-9].