How to join by two fields showing the not matching records and group by using Linq

Question

In Visual Studio 2013, C# 4.5, I have these classes:

Entities:

public class KpiLabourHoursByTeamRole
{
    public DateTime Date { get; set; }
    public int TeamId { get; set; }
    public int RoleId { get; set; }
    public double TotalHours { get; set; }
    public double TotalCost { get; set; }
}

public class TeamRole
{
    public int Id { get; set;}
    public int RoleId { get; set;}
    public int TeamId { get; set;}
    public int? LabourCostCenterId { get; set; }
}

public class KpiLabourHoursByCostCenter
{
   public int? LabourCostCenterId { get; set; }
   public double TotalHours { get; set; }
   public double TotalCost { get; set; }
}

Current Solution:

Currently, I have the solution below working perfectly as a Inner Join using Linq:

kpiLabourHours is a object of the type: KpiLabourHoursByTeamRole

kpiLabourHoursByCostCenter is a object of the type: KpiLabourHoursByCostCenter

var kpiLabourHoursByCostCenter = from k in kpiLabourHours
    join tr in teamRoles on new {k.TeamId, k.RoleId} equals new {tr.TeamId, tr.RoleId}
    group k by new { tr.LabourCostCenterId }
    into grp
    select new KpiLabourHoursByCostCenter
    {
        LabourCostCenterId = grp.Key.LabourCostCenterId,
        TotalHours = grp.Sum(k => k.TotalHours),
        TotalCost = grp.Sum(k => k.TotalCost),
    };

Equivalent in Sql:

The code above is represented by the equivalent Sql:

SELECT tr.LabourCostCenterId, sum(k.TotalHours), sum(k.TotalCost)
FROM kpiLabourHours k
INNER JOIN TeamRoles tr on tr.TeamId = k.TeamId and tr.RoleId = k.RoleId  
GROUP tr.LabourCostCenterId

My problem:

The problem is that it is hidding the results where there is no matching TeamId and RoleId, so I need to replace this code by a kind of Left Outer Join, such as:

SELECT tr.LabourCostCenterId, sum(k.TotalHours), sum(k.TotalCost)
FROM kpiLabourHours k
LEFT OUTER JOIN TeamRoles tr on tr.TeamId = k.TeamId and tr.RoleId = k.RoleId  
GROUP tr.LabourCostCenterId

In a nutshell, I need to return LabourCostCenter NULL or 0 if there is no matching TeamId and RoleId with the totals.