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Fig. 2


The position of Polaris on the predicted reddening-free Period-Wesenheit relation for FO Cepheids on the first crossing; relations for models on the blue and red IS edges are shown in these colors. Solid lines are model predictions featuring average rotation (half of critical angular rotation rate on ZAMS). Models without rotation or very fast rotation are shown by dotted and dash-dotted lines and are very similar near the blue edge. SU Cas is another FO Cepheid with positive (likely on second crossing) and available parallax (van Leeuwen 2007, HIPPARCOS). PLRs near red IS boundaries for second and third crossing FO Cepheids are shown in cyan and yellow. Blue boundaries for second or third crossings could not be established at such short periods, since their blue loops are not extended to sufficiently high temperatures (cf. Anderson et al. 2017).

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