WPF remove vertical line between the ribbongroups

Question

I am trying to remove the vertical line between the ribbongroups. I have added an image that shows what I mean. The thing I am trying to get rid of is highlighted in red. I cannot find any info on this online.

Any comments or notes are helpfull!

Code sample for a test application that shows the same behaviour:

<Window x:Class="TestRibbon.TestWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:r="clr-namespace:Microsoft.Windows.Controls.Ribbon;assembly=RibbonControlsLibrary"     
    Title="TestWindow" Height="300" Width="300">
<Grid>
    <r:Ribbon>
        <r:RibbonTab Name="Test" Label="Test">
            <r:RibbonGroup>
                <r:RibbonButton>

                </r:RibbonButton>                    
            </r:RibbonGroup>
            <r:RibbonGroup>
                <r:RibbonButton>

                </r:RibbonButton>
            </r:RibbonGroup>
            <r:RibbonGroup>
                <r:RibbonButton>

                </r:RibbonButton>
            </r:RibbonGroup>
            <r:RibbonGroup>
                <r:RibbonButton>

                </r:RibbonButton>
            </r:RibbonGroup>
        </r:RibbonTab>

    </r:Ribbon>
</Grid>

PS. I am using a standard WPF application, no themes. .net 4.5