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This article says that the companion star of the pulsar PSR B1516+02B must be “tiny” because it cannot be seen. Isn’t it possible that the companion is made of dark matter? Is there a “wobble” test or other way to discern between a companion that is truly tiny (low mass) and one that is perhaps more massive but not visible? The mass of the companion star seems to bear directly on the conclusion favoring heavy neutron stars.

Maureen McAllister
Wayne, Ill.

The companion to PSR B1516+02B is very likely a white dwarf star, says Paulo Freire of Arecibo Observatory in Puerto Rico. White dwarfs are faint, and become harder to detect as they grow older and cooler. Few companions to millisecond pulsars have ever been directly detected, but those that have are almost all white dwarfs. Freire adds that dark matter seems to clump at galactic scales, and that he is not aware of any theoretical predictions of dark matter able to form small objects like stars. —Ron Cowen