Background: Traceability between software artifacts enhances the value of the information those artifacts contain, but only when the links themselves are reliable. Link quality is known to depend on explicit factors such as the traced artifacts and the expertise of the practitioner who judges each connection. Other factors, however, remain largely unexplored. We contend that one of these factors is the set of auxiliary artifacts -- artifacts that are produced and/or used during the tracing process yet are neither the source nor target artifacts. Because such auxiliary artifacts can subtly steer how links are created and validated, they merit a literature survey to identify these artifacts and further investigate them. Objective: We identify and map auxiliary artifacts used in requirements tracing, which could be additional factors that affect the quality of the trace links. Method: We conducted a systematic mapping study on auxiliary artifacts in requirements traceability. Results: We found 110 studies in which auxiliary artifacts are used in requirements tracing, and identified 49 auxiliary artifacts, and 13 usage scenarios. Conclusion: This study provides a systematic mapping of auxiliary artifacts in requirement tracing, including their usage, origin, type and tool support. The use of auxiliary artifacts in requirements tracing seems to be the norm, thus, these artifacts should be studied in depth to identify how they effect the quality of traced links.