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We report the discovery of the millisecond pulsar (MSP) PSR J1950+2414($P=4.3$ ms) in a binary system with an eccentric ($e=0.08$) 22-day orbit inPulsar ALFA survey observations with the Arecibo telescope. Its companion starhas a median mass of 0.3 $M_\odot$ and is most likely a white dwarf. Fullyrecycled MSPs like this one are thought to be old neutron stars spun-up by masstransfer from a companion star. This process should circularize the orbit, asis observed for the vast majority of binary MSPs, which predominantly haveorbital eccentricities $e < 0.001$. However, four recently discovered binaryMSPs have orbits with $0.027 < e < 0.44$; PSR J1950+2414 is the fifth suchsystem to be discovered. The upper limits for its intrinsic spin periodderivative and inferred surface magnetic field strength are comparable to thoseof the general MSP population. The large eccentricities are incompatible withthe predictions of the standard recycling scenario: something unusual happenedduring their evolution. Proposed scenarios are a) initial evolution of thepulsar in a triple system which became dynamically unstable, b) origin in anexchange encounter in an environment with high stellar density, c) rotationallydelayed accretion-induced collapse of a super-Chandrasekhar white dwarf, and d)dynamical interaction of the binary with a circumbinary disk. We compare theproperties of all five known eccentric MSPs with the predictions of theseformation channels. Future measurements of the masses and proper motion mightallow us to firmly exclude some of the proposed formation scenarios.

Einstein@Home Discovery of a PALFA Millisecond Pulsar in an Eccentric Binary Orbit