Universal triple Massey products on elliptic curves and Hecke's   indefinite theta series
  Generalizing math.AG/0003076 we express universal triple Massey products between line bundles on elliptic curves in terms of Hecke's indefinite theta series. We show that all Hecke's indefinite theta series arise in this way. 
Vicentiu Pasol, Alexander Polishchuk