  We develop and apply methods to extract planet masses and eccentricities from
observed transit time variations (TTVs). First, we derive simple analytic
expressions for the TTV that include the effects of both first- and
second-order resonances. Second, we use N-body Markov chain Monte Carlo (MCMC)
simulations, as well as the analytic formulae, to measure the masses and
eccentricities of ten planets discovered by Kepler that have not previously
been analyzed. Most of the ten planets have low densities. Using the analytic
expressions to partially circumvent degeneracies, we measure small
eccentricities of a few percent or less.
