Keifer, thanks for the swift answer,

! shift work

I followed https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/obfs4proxy

on a vps with Debian 9
the torrc reads

RunAsDaemon 1
ORPort 9001
BridgeRelay 1
ServerTransportPlugin obfs4 exec /usr/bin/obfs4proxy
ExtORPort auto
DataDirectory /var/lib/tor
ContactInfo <p at gmail dot com>
Nickname <name>

from root this

tail -F /var/log/tor/log

brought this

tail: cannot open '/var/log/tor/log' for reading: No such file or directory

Rob