Quasi-periodic oscillations in accreting magnetic white dwarfs - I. Observational constraints in X-ray and opticalJ. M. Bonnet-Bidaud, M. Mouchet, C. Busschaert, E. Falize and C. MichautA&A, 579 (2015) A24DOI: https://doi.org/10.1051/0004-6361/201425482