def get_forecast_metadata_variables(ds):
    """
    Returns a list of variables that represent forecast reference time
    metadata.

    :param netCDF4.Dataset ds: An open netCDF4 Dataset.
    :rtype: list
    """
    forecast_metadata_standard_names = {
        "forecast_period",
        "forecast_reference_time",
    }
    forecast_metadata_variables = []
    for varname in ds.variables:
        standard_name = getattr(ds.variables[varname], "standard_name", None)
        if standard_name in forecast_metadata_standard_names:
            forecast_metadata_variables.append(varname)
    return forecast_metadata_variables