I have analyzed the spin period for the intermediate polar V405 Aur using all data from AAVSO and also data from articles by Breus and Piirola. The result is an O-C diagram showing phase vs HJD covering the time from 1994 to 2024. The diagram is a bit noicy until 2012 (uncertainties and/or errors in the early data?). After 2013, which only contain AAVSO data, the results are much more in accordance.
From 2014 and onward the star seems to have changed its behaviour and the spin period seems to increase in a steady pace. I have got the pace to 0.00011 seconds/year or 2.28E-14 days/cycle. I don't know if this has been noticed before for this star?
Images of the O-C diagrams for 1994-2024 and 2014-2024 are available on the swedish astro forum Nattmolnet at https://nattmolnet.saaf.se/forums/topic/v405-aur/#post-7346
Thomas Karlsson