Can I get co-founder equity when leaving a startup?

As a graduate student, I worked over a period of 6 months together with a co-founder in a startup. I invested roughly 25% of my time. The co-founder funded the startup and worked full-time. I was supposed to join full-time as well and we would split 50/50, but we missed a milestone and I am leaving to look for a regular job. Among other things I have worked on website optimization, data gathering & analysis, brainstorming w/ co-founder, marketing material, and editing an accelerator application. Upon leaving, I am wondering if I should receive a few percentage points of co-founder equity in the startup, or if this is unfeasible. When the startup launched we signed no formal contract.