The design of the variability of a software product line is crucial to its success and evolution. Meaningful variable features need to be elicited, analyzed, documented and validated when an existing software or reference system evolves into a software product line. These variable features are the main discriminators between individual products and they need to reflect the needs of a large variety of stakeholders adequately. In this paper we present a novel approach, called feature unweaving, that supports the identification and extraction of variable features from a given graphical software requirements model. We have extended our aspect-oriented software product line modeling tool   such that it supports feature unweaving: it takes a set of model elements that a domain requirements engineer considers to constitute a variable feature and automatically refactors the model into a semantically equivalent one in which the model elements belonging to this feature are grouped into an aspect. This allows the identification and modeling of variable features in an incremental style. It also substantially reduces both the intellectual and clerical effort required for constructing the variable parts of a software product line requirements model.