Feature-oriented programming (FOP) is an extension of object-oriented programming to support software variability by refining existing classes and methods. In order to increase the reliability of all implemented program variants, we integrate design by contract (DbC) with FOP. DbC is an approach to build reliable object-oriented software by specifying methods with contracts. Contracts are annotations that document and formally specify behavior, and can be used for formal verification of correctness or as test oracles. We present and discuss five approaches to define contracts of methods and their refinements in FOP. Furthermore, we share our insights gained by performing five case studies. This work is a foundation for research on the analysis of feature-oriented programs (e.g., for verifying functional correctness or for detecting feature interactions).