A New Algorithm for Maximum Likelihood Estimation in Gaussian Graphical   Models for Marginal Independence
  Graphical models with bi-directed edges (<->) represent marginal independence: the absence of an edge between two vertices indicates that the corresponding variables are marginally independent. In this paper, we consider maximum likelihood estimation in the case of continuous variables with a Gaussian joint distribution, sometimes termed a covariance graph model. We present a new fitting algorithm which exploits standard regression techniques and establish its convergence properties. Moreover, we contrast our procedure to existing estimation methods. 
Mathias Drton, Thomas S. Richardson
stat.ME cs.LG stat.ML