Co-crystals of caffeine and piracetam with 4-hydroxybenzoic acid: Unravelling the hidden hydrates of 1: 1 co-crystals

Attempts to prepare 1 : 1 co-crystals of caffeine and piracetam with 4-hydroxybenzoic acid have led to the serendipitous discovery of hydrates of a previously unknown 1 : 1 co-crystal of caffeine and 4-hydroxybenzoic acid, and a known 1 : 1 co-crystal of piracetam and 4-hydroxybenzoic acid.