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Criteria for IP ClassificationOn this site, I have the absolute power to decide which systems I include as IPs, which as candidates. Now I have started a 5-tier system from "ironclad" to "doubtful." "Ironclad" is used for systems that so obviously belong to the class, there are no arguments (e.g., AO Psc), while "confirmed" systems may not have as many independent lines of evidence, or one or more of the periods may not be known, or known accurately. "Probable" and "possible" indicate the lack of hard evidence either way, but that I have my personal guesses as to whether they are IPs ("probable") or not ("possible")."Doubtful" usually means there are later papers that disagree with the IP classification. My philosophy for this catalog is to never remove an object if it was once proposed to be an IP in refereed journal, even if there is a new consensus that it is not (including objects that are not even CVs). Joe's 1994 review contains a section titled "The Cast: Rules of the Audition." Of his 6 criteria, I would not be bothered by a lack of circular polarization or a relatively soft X-ray spectrum - there are confirmed IPs with neither. However, I place a stronger emphasis on optical emission lines. Penning's pioneering work as well as many papers by Coel Hellier and company show that IPs have radial velocity (or its proxy, V/R ratio) modulation on the spin period. This is easy to underatsnd: a significant fraction of optical emission lines in IPs come from the "accretion curtain." So, if time-resolved optical spectroscopy with a sufficient sensitivity fails to show such a behavior, I would take that as a strong negative. Of course, the usual criteria for IP classification is optical and X-ray spin modulations. When the classification depends on the optical periodicity, it should be coherent and persistent to a high degree. It has to be repeatedly observed, ideally over several years; If you see apparent modulations for a few cycles --- don't get too excited. I think every CV observer should read Warner's Warning (or see th PDF version of the same) before attempting to identify periodicities in CVs. If I were writing my papers on KO Vel now, with the same data, I would probably write a very different paper. When X-ray pulses are involved, I would relax the requirement --- two independent X-ray pulse period determinations would be sufficient for me (assuming it is known to be a CV and the periods agree). It is not sufficient for a sub-orbital X-ray periodicity to be seen once. Such one-time-only X-ray periodicities have been seen in many non-magnetic CVs. Usually in such systems, different periods are seen in different X-ray observations, proving that these X-ray modulations are not tied directly to the spin period of the white dwarf. I would also happily accept somewhat weak optical detection (in terms of establishing persistence and coherence) combined with a single X-ray detection of the same period.
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