Digital Twins are a major driver in the digital transformation of industry. It is gaining in adoption in industry and interest in research and standardization. One of the characteristics of digital twins is the coupling and synchronization between the asset and its virtual representation, raising the issue of interoperability. To date, there is no unique reference architecture for digital twins, nor any implementation covering the full range of capabilities needed to implement digital twins for industrial systems, which may in practice pose a barrier to their adoption and dissemination. Our approach is to move towards an open platform aligned with standards to promote interoperability. The contributions of the paper are twofold: i) a proposal for a high-level architecture of a standards-compliant digital twin platform; ii) a proposal for an open-source implementation on the restricted perimeter of asset/digital twin coupling. This implementation is aligned with the IEC 63278 standard and the Asset Administration Shell vision of Industry 4.0. The article presents 19 criteria applied to the selection of existing open-source implementations, as well as the integration of these into a technical architecture and its implementation.