  The focus of this paper is on identifying the most effective selling strategy
for pairs trading of stocks. In pairs trading, a long position is held in one
stock while a short position is held in another. The goal is to determine the
optimal time to sell the long position and repurchase the short position in
order to close the pairs position. The paper presents an optimal pairs-trading
selling rule with trading constraints. In particular, the underlying stock
prices evolve according to a two dimensional geometric Brownian motion and the
trading permission process is given in terms of a two-state {trading allowed,
trading not allowed} Markov chain. It is shown that the optimal policy can be
determined by a threshold curve which is obtained by solving the associated HJB
equations (quasi-variational inequalities). A closed form solution is obtained.
A verification theorem is provided. Numerical experiments are also reported to
demonstrate the optimal policies and value functions.
