Old French subject pronouns (Spro) were omissible if postverbal (Foulet 1928), but not freely so (Vance 1997, Zimmermann 2014). This article addresses their partial omissibility in discourse-syntax terms, following work on partial null subject languages by Holmberg and Nikanne (2002) and Modesto (2008). An observational study of dialogic responses in 13th century prose romances is first reported, finding strong indications of covariation between the Topic/Focus status of an initial non-subject constituent and the expression/omission of post-verbal Spro. A quantitative investigation, in such texts, of preposed discourse-linked anaphoric constituents and preposed intensifiers, taken as diagnostic of Topichood and Focushood respectively, confirmed this analysis. We take null Spro to be available (i) when a null Topic operator targets left-peripheral TopicP, and (ii) with a left-peripheral Focused expression. When a discourse-linked non-subject constituent occupies TopicP, however, Spro must be overt.