Long-term semiregular dust formation by the WC9+B0I system WR 70

1 Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ; 2 Space Research Organization Netherlands, Sorbonnelaan 2, NL-3584 CA Utrecht, The Netherlands; 3 Astronomical Institute `Anton Pannekoek', University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, The Netherlands; 4 South African Astronomical Observatory, P.O. Box 9, 7935 Observatory, South Africa; 5 Astronomy, Cosmology and Gravitation Centre, Astronomy Department, University of Cape Town, 7700 Rondebosch, South Africa; 6 Service d'Astrophysique DSM/IRFU/SAp CEA Saclay, L'Orme des Merisiers, b^atiment 709, F-91191 Gif-sur-Yvette France; 7 Atacama Large Millimetre/submillimetre Array (ALMA), Alonso de Cordova 3107, Vitacura, Santiago 763 0355, Chile