Looks like Biostar accidentally leaked some details about its upcoming Racing Z390 series. A manual that ships with the company's Racing B360 series contains details about the unreleased Racing Z390 GT3 and Racing Z390 GT5 motherboards.
It's believed that the Intel Z390 chipset as well as eight-core Coffee Lake-S processors will be released in the coming months. The fact that Biostar already has these materials ready suggests these products may be coming sooner rather than later.
The manual can be downloaded from Biostar website and its content are not really giving us much information about the Z390 chipset itself. It does mention though that B360 chipset does not allow overclocking while Z390 does. The motherboard also supports CPUs up to 95W, which may indirectly confirm the maximum TDP of 8-core Coffee Lake-S CPU.